AREA TO RELEASE BRAKES

SPECIFICATION This application relates to a fluid powered, hydraulic brake system, and more particularly to a master cylinder assembly therein having a main spring and pressure area to apply brakes, and an auxiliary spring and differential area to release the brakes.

Power brake boosters, according to practice in the past, hydraulically work against the same friction brake return springs as the operator-operated brake pedal, and so they assist the latter only before brake release, whereupon the hydraulic pressure is released at the power source and the springs forthwith return the brakes and booster and pedal to their unapplied positions. It is therefore the practice to use up to full pressure of the power source only during brake application.

But the need of fuller utility deserves to be recognized, and how I might go about satisfying that utility. One need, already known in some brake applica¬ tions differing herefrom, is to utilize the power source pressure additionally to maintain a retractor chamber or other accumulator at full pressure as an emergency power braking reserve for the booster. More significantly, the

power source can be made to find a second and a third im¬ portant utility either from the application, when desired, of up to the full pressure thereof to apply brakes or from application, when desired, of the full pressure thereof to release the brakes .- My invention in accordance with one embodiment incorporates a distinctive differential piston plunger in the master cylinder and, in accordance with the principles of my invention, each utility of the foregoing three needs is recognized and is in fact satisfied by the differential piston plunger.

As will be explained hereinafter in detail, the plunger affords the referred to main pressure area in the cylinder which it presents when it makes its brake applying motion as augmented by the mainspring, and also affords the referred to differential pressure area which it presents to receive the force of the referred to auxiliary power spring and auxiliary spring powered retractor chamber in order to release the brakes. More specifically in connection with the stepped plunger and its differential pressure area, the auxiliary spring presses on a power element which is operated thereby in the retractor chamber, and the power element consists of a piston which can be, but not neces¬ sarily is, also of stepped construction.

The principal and overriding attributes to having the main and differential pressure areas present, however, are that the brake-apply, main area is the precise thing which can be availed of to be pressure movable in response to up to full pressure of the fluid supply source, and the brake-release, differential area is the precise thing which can be availed of to be pressure movable in response to up to full pressure of that same source. These two alter¬ nately active areas allow the system to be power operated in a manner to which the term bi-powered is believed aptly applied in a paragraph which is to follow shortly herein. Background patents, identified because of their relevance to differential pistons in brakes and in further but nonanalogous applications, include but are not limited

to U.S. nos. 2,532,801, 2,991,758, 3,242,825, 3,462,986, 3,522,760, 3,896,706, and, at least ostensibly so, the very recent nos. 4,253,306 and 4,256,017.

One or more of those background patents belong primarily with the art in the Class 92 subclasses, which are primarily devoted to auxiliary spring powered safety actuator patents in the relevant brake art such as the ad¬ ditional U.S. nos. 2,992,630, 3,576,152 and 3,911,795. It is an object of my invention foregoing to provide a system which is bi-powered in the respect that full power releases the brakes whereas up to full power also applies the brakes .

Another of my objects is provision of a system which is bi-hydraulic in the respect that power to hydrau¬ lically brake is applied hydraulically.

An additional object is that my invention provides a bi-modal system in respect to power brake actuation that transpires in two modes or stages which are entirely separate in character but which blend smoothly together.

^■ A related object is the provision of a system which is bi-mc5dal in respect to emergency operation.

A further related object is to provide a system bi-modal in respect to another vehicular component con¬ trolled at the same time, specifically, brake applica¬ tion with or without automatic neutralization of the trans¬ mission.

A further object resides in the provision of a bi-redundant system as a safeguard in respect to valve linkage failure, and to backup mechanisms which can be immediately availed of by the operator.

One additional object is to provide a system which is bi-cylinder in the respect that the axles, included in a dual braking system, are separately hydraulically braked by independent master cylinders which are included in the system.

One further object of my invention is to provide a device of compact design, with its master cylinder and foot valve and booster combined into the one device.

It is a further object to provide a pump powered brake system instantaneous in response to setting the brakes, which yet does not draw much oil from the pump. Another object, with the power brake function which I provide in addition to the regular service brakes, is that the regular service brakes function as the emer¬ gency brake used for stopping in an emergency so that a third brake, the regular parking brake, can be of small design when provided. Another object is the provision of an operator- operated power brake in which, in case of power failure, a number of applications, for example one or more, can be made with or at least can be assisted by emergency power, and then an indefinite number of applications can be made by the operator, unassisted, and analogously to the straight through braking operation of other systems.

Further features, objects and, advantages will either be specifically pointed out or become apparent when, for a better understanding of the invention, reference is made to the following description taken in conjunction with the accompanyiaig drawings which show a preferred embodiment thereof and in which:

FIGURE 1 is a longitudinal cross sectional view of a master cylinder assembly shown in its normal vehicular environment in a full power, brake release mode thereof and embodying the present invention;

FIGURE 2 is a schematic showing of the same parts of the brake master cylinder assembly in their released, emergency assist mode; FIGURE 3 schematically shows the parts in re¬ leased, no assist mode of operation;

FIGURE 4 shows the same parts schematically, in their applied, no assist mode;

FIGURE 5 is similar to Figure 1 but shows the parts in a condition corresponding to the brake being barely set by spring pressure under power control;

FIGURE 6, similar to Figure 5, is a showing cor¬ responding to a position with the brake set with full spring pressure, under power control;

FIGURE 7 is a schematic view of the full power, brake applied condition;

FIGURE 8 graphically shows the source pressure/ brake torque relationship throughout a brake pedal stroke; and

FIGURE 9 shows a detail of Figure 1 modified for a hand lever back-up operation adding redundancy to the brake operating system.

Certain draf sman' s liberties have been taken with the master brake cylinder power assembly 10 hereof, in con¬ solidating different views all in the single Figure 1. Figure 1 centers on a rectangular box housing 12 appearing in a front and side, multiple view, elevational showing 12a and 12b in which, at the front, this housing for the power brake assembly is broken away at 12a to expose an auxiliary differential piston 14 and a differential valve 16 at the front having a vertical stroke axis 18 in common, as seen in this figure. Complementarily, the housing 12 at the side is shown broken away in its appearance at 12b to expose, within a dual cylinder extension 36, a differential piston plunger 24 at the rear having a horizontal stroke axis 26.

Moreover, in the brake assembly environment ap¬ pearing marginally in Figure 1, the rubber tired vehicle 20 equipped as illustrated with brakes 22 requiring the power operation, has a power shift transmission appearing therein in the solid line showing 28a and again appearing in the right margin of Figure 1 in the broken line showing 28b. Also, the operator's brake pedal assembly for the vehicle 20 appears therein in the broken line showing 30a and again in the bottom margin of Figure 1 in the solid line showing 30b.

As an additional prefatory remark, considerable redundancy can also be said to be present herein, built

into the actual system's operation, to safeguard the operator with different routes of control which can be availed of at will in emergencies, possible from hydraulic or mechanical breakdowns such as power failure, linkage failure, control valve failure, and so forth. Finally it is in order, by way of early explanation, to point out that each of the front and rear vehicle axles 32 and 34 is separately controlled by its own master cylinder 36a and 36b in extension 36 for the appropriate brakes 22, which latter in operation are redundant to one another at front and rear.

PARTICULARS — FIGURE 1 Differential piston plungers, which are provided slidably received in the cylinders of the dual cylinder extension 36, are identical to one another and only the differential piston plunger 24 is illustrated. The dif¬ ferential piston plungers are controlled by the differ¬ ential valve 16 appearing in the showing 12a and, through separate hydraulic brake lines 38a and 38b, the plungers control the vehicle brakes 22.

The plunger proper 40 of the differential piston plunger 24 is shown in full retracted position for brake release, and nas a relief 42 at the end and a passage 44 for a one-way ball check valve. The ball check passage 44 constantly communicates through a port 46 with a reservoir of hydraulic fluid, not shown, and thus prevents the plunger proper 40, as it retracts into the illustrated posi¬ tion, from drawing a vacuum in the cylinder 36a. The plunger relief 42 at end of its retraction stroke equalizes the pressures between the reservoir and the brake cylinder 36a because of the permanently open restrictive port 48 between the relief and the reservoir.

The differential piston plunger 24 as a step 50, and a piston portion 52, of large diameter carrying a seal 54, is divided thereby from the plunger proper 40 of smaller diameter carrying seals 56.

The resulting pressure movable differential area 58 travels in a cylindrical enlargement of the cylinder 36a defining a differential area pressure chamber 60. A main area chamber 62 provides the volume in which the main pres- sure movable area 64 of the differential piston plunger 24 moves, and it will be seen that the latter has a hollow interior 66. This hollow interior 66 constitutes a spring chamber receiving a mainspring 68 and, if desired, a light spring 70 thereabout to augment it. A differential chamber 60 is pressurized with fluid such as oil from the modulated release-pressure inlet-outlet line PI and the main area chamber 62 is pressurized with fluid from the modulated apply-pressure inlet-outlet line P2. A drain D connected between adjacent ones of the seals 56 ensures that the fluid of chamber 60 and the brake fluid in cylinder 36a do not collect to commingle with one another; neither leakage, if any, past a seal is allowed to accumulate within the extension 36.

A differential pressure sensing switch 72 con¬ nected across the hydraulic brake lines 38a and 38b is included in a*brake failure indicator circuit having a lamp 74 on the vehicle instrument pannel which lights if either of the two lines, which should stay equal, loses pressure relative to the other line. The lamp 74 then functions, in known way, to remain illuminated thereafter even when the brakes are idle. Although a dirigible wheel construction can be provided for the front wheels 76 so as to render them steerable relative to the rear ones 78 in the vehicle 20, the vehicle illustrated has articulated-section steering and the respective sections carry the described fixed axle 32 and fixed ascle 34 at front and rear. The front axle 32 is braked at 22 by hydraulic brake pressure from line 38a, whereas the rear axle 34 is braked at 22 by hydraulic pressure from the brake line 38b connected to the separate master cylinder 36b.

The braking is controlled by the assembly 30a which is pedal operated as will be hereinafter described in detail.

In its showing appearing at 12a, the power brake assembly receives, and the transmission in its appearance as shown by the broken lines 28b delivers, the pressure fluid not only for operating the power shift transmission but also for operating the power braking. More particular¬ ly, the transmission 28b drives a pump source 80 of the pressure fluid at an actual point on the transmission more conventional and appropriate than the schematically con¬ venient rear end point shown. The pump source 80 delivers regulated pressure in an outlet line 82 supplying a header 84. The assembly housing 12a has a second bore core 86 which is supplied by the header 84 with fluid under the regulated line pressure P and. which is blocked by the differential valve 16 in its brake-release position shown.

A first bore core 88 receives the flow of fluid under line pressure P from the header by way of a check and restriction assembly 90, and delivers the flow through a modulated release-pressure line PI so as to be applied directly to the differential chamber 60. This flow at the time occurs with undiminished line pressure P, because a ^* seated ball regulator valve 92 in the position as illustrated is being held at that time firmly on its seat in the differential valve 16 by a calibrated regulating spring 94; when active, the valve 92 regulates pressure in line P^ down to reduced values.

A retractor chamber 96 is normally kept pres¬ surized at line pressure P through a check and restriction assembly 98 leading from the header 84, and has a single outlet effective only when the differential valve 16 is in the full release position as illustrated. Retractor chamber pressure acts against the differential area 100 of differential valve 16 with constant bias.

More particularly, the referred to single outlet for the retractor chamber 96 is provided by surface relief

of the valve starting at differential area 100 thereon and consists of several short longitudinal valve slots 102 establishing a manner of direct communication between the chamber 96 and the first bore core 88. In that manner, the retractor chamber 96 also directly communicates with the differential chamber 60 by way of the modulated release- pressure inlet-outlet line PI. Thus so long as full line pressure P is made available by the valve 16, the differ¬ ential piston plunger 24 is held in the brake release position illustrated because its differential chamber 60 is pressurized by the manifold 84 through two separate routes 102 and 90.

The main area 104 of the auxiliary differential piston 14 is confrontingly in the path of the differential area 100 of the valve 16 and either of the two can move across chamber 96 into engagement with the other. The piston 14 slides with its bored interior riding on the small diameter portion of the valve 16 which is formed in the sliding area with conventional circular hydraulic balancing grooves 105.

The "auxiliary piston 14 is a differential piston in name only, and its differential area 106 is merely a spring seat for a heavy auxiliary brake spring 108 occupy¬ ing a spring chamber in the cylinder ,12a. The spring 108 reacts against an end cover 110 of the power brake assembly housing and can be supplemented in its force by a light secondary auxiliary spring 112 mounted mechanically in parallel with the spring 108. The secondary spring 112 is concentrically located between the spring 108 and an inner stop sleeve 114 for the piston 14 which slidably receives the differential valve 16.

A singletree having fulcrum ends 118 and 120 to the left and to the right, respectively, as viewed in Figure 1 is connected by a pivot 122 at the mid section for exerting a pull under leverage to operate the differential valve 16. The singletree 116 is actuated by pulling the left fulcrum end 118, for example, through a pedal connec-

tion 124 to a brake pedal 126 included in the pedal assembly 30b.

The full power, brake released condition of the assembly is illustrated in Figure 1 only.

RELEASED, EMERGENCY ASSIST — FIGURE 2 The differential piston plunger 24 appears in this figure the same as before, fully released. But because of hydraulic pump power failure, presumed for purposes of this illustration, the auxiliary spring actuated auxiliary piston 14 is shown in a position corresponding to about two quarters of its stroke compared to the relative four quarters illustrated in Figure 1, thus collapsing the retractor chamber 96 accordingly sufficiently to fill the differential chamber 60 and fully release the plunger 24. Under hydraulic failure of the pump source, the automatically collapsing movement' of the auxiliary piston 14 will account for more than one emergency power assisted brake release depending upon how much braking was effected and needed .to be overcome in the system. In one physical example, where the release was required from full braking, the emergency fluid in retractor chamber 96 calculated out to be of sufficient volume for one and a half emergency assisted brake releases.

RELEASED, NO ASSIST — FIGURE 3

Operator operated full releases of the plunger 24 into the position illustrated in this figure can be re¬ peated indefinitely after the emergency pressurized volume of pressure fluid has been exhausted from the retractor chamber 96. In distinction to the showing of Figure 2, however, the auxiliary piston 14 in.the Figure 3 showing has a position corresponding to about one quarter of a full stroke but, with that stroke margin, still ensures full brake release after each simple foot operation of the brake pedal to recompress the spring 108 and then release.

Under such no assist condition, the reason for the difference noted is that that auxiliary piston 14 starts off for release at a position corresponding only roughly to three quarters of a full stroke for reasons now to be explained.

APPLIED, NO ASSIST — FIGURE 4 During brake application in this figure as illus¬ trated therein at 24 with no power, the operator is re- quired by foot effort alone to advance the differential valve 16 sufficiently to later recharge the chamber 96, at the same time engaging with its differential area 100 the auxiliary piston 14 to move it back to what would be equiv¬ alent to about three quarters of a full stroke. The re- sisting strength of the heavy auxiliary spring 108 limits how far the operator by foot effort alone can retract the piston 14 and, on average, the static force on the pedal required from pressing down by foot is about 125 pounds (56 kg.). Thereafter, and as previously discussed in connec¬ tion with Figβre 3, the operator can release the foot pres¬ sure on valve 16 whereupon the auxiliary spring actuated piston 14 will collapse the chamber 96 for full release of the brakes. The foregoing emergency operation, with and with¬ out emergency power reserve, is bi-modal and, without the reserve, the operator operated brake applications can be repeated an indefinite number of times, for example, thirty or more. Power operation is only slightly more involved as the following paragraphs will bring out; it too is bi- modal, allowing the brakes to be applied progressively but in two separate successive stages.

BRAKE BARELY SET BY SPRING PRESSURE

UNDER POWER CONTROL — FIGURE 5 Hardly more than a tap on the brake pedal 126 is required to give the differential piston plunger 24 move-

ment into its predetermined initial braking position as illustrated in this figure. With such initial pedal movement the pull connection 124 swings the end 118 of the singletree 116 about the end 120 which is against the end 5 cover 110 and which pivots on the cover as fulcrum. Under the full line pressure P, the retractor chamber 96 is fully expanded so that the closed slots 102 hold the differential piston 14 immobile but in standby readiness in its full emergency position, described as the four quarters or full 10. stroke of retraction.

Certain further structure of the differential valve 16 takes on significance when the pivot 122 connected to the singletree 116 causes such predetermined initial movement of the valve 16. The portion concerned is the

15 hollow, large diameter extremity of the valve. First, the valve 16 begins telescopic disengaging movement from a slug 128 therein located as a core at the end of the valve bore 130 of the assembly housing. A drain port 132 through the hollow wall in the side of the valve 16 uncovers a portion 0 of a slug shoulder 134 serving as an anchor for the cali¬ brated regulating spring 94. The port 132 thus opens to drain pressure the hollow interior 136 of the valve in a path through a spring chamber 138 thence into a refill standpipe reservoir generally indicated at 140 and ex- 5 tending upwardly and over to a connection to a drain D.

The spill-over enroute to drain D will ensure that a long vertical standpipe 141 stays full leading to the spring chamber below piston 14.

Second, consequent elongation of the regulating 0 spring 94 allows the ball regulator valve 92 to start regulating back pressure on the restricted flow of fluid into the first bore core 88 by reason of the connection to the latter of the valve seat through the axial and radial passages 142 in valve 16. So, modulated release-pressure 5 inlet-outlet line PI reduces the equivalent pressure PI of differential chamber 60, enabling the mainspring 68 and augmental spring 70 to start the plunger 24 in the brake apply direction in the cylinder 36a.

Release of the pedal 126 will restore all parts to the full power, brake release positions as shown and al¬ ready described in connection with Figure 1.

CLOSED SYSTEM STROKE — FIGURE 5

Although this figure does not depict emergency un¬ assisted circumstances, such circumstances can readily be visualized therefrom as the valve 16 is foot forced farther downwardly, past the point illustrated, for full applica- tion of the brakes in an emergency during hydraulic power failure.

Under these circumstances, the auxiliary differen¬ tial piston 14 would not be at the illustrated point but should be visualized as enough higher in the cylinder than appears, so as to be in engagement against, and moving downwardly with, the abutting differential area 100 of the valve. A check valve 143 connected to the reserve volume maintained in the refill standpipe reservoir 140 immedi¬ ately unseats to replenish the retractor chamber 96 with fluid for the volume being vacated therein by the with¬ drawing face -f*04 of the piston 14.

At the same time, because the ball regulator valve 92 is off its seat, an exactly equal volume is being trans¬ ferred from the collapsing differential chamber 60 right back into the always maintained, reserve fluid body in the refill standpipe reservoir 140. In fact, most if not all of the oil of the collapsing chamber 60 transfers directly through the check valve 143 into the chamber 96 as it is being manually expanded.

COUNTERPART STROKE — FIGURE 3

Spring forced movement of the piston 14 into the released, no assist position as shown this figure reuses the amount of the volume just noted to have been trans- ferred into the retractor chamber 96, by retransferring same in toto through the valve control charge path 102, not shown, to cause operation under pressure in chamber 60 of

the plunger 24 in a chamber vacating, brake release direction.

The cycle is then repeated. That is to say, again foot forcing the differential piston 14 to reevacuate the chamber 96 and foot forcing the valve 16 to project into its brake apply position, affords flow, from the differ¬ ential chamber 60 through the valve control release path 92, not shown, of a volume to reservoir of the same volume as being vacated by motion of the downmoving differential piston 14 as viewed in Figure 3.

Because of this novel refill standpipe reservoir circuit design, there is no net loss of oil allowed to go from the reservoir 140 down into the standpipe 141 as shown in Figure 5 or to go on over into the drain D. Thus the auxiliary differential piston 14 during emergency condi¬ tions can charge the chamber 96 thereabove and refill the spring chamber therebelow from the refill reservoir circuit 140 and the standpipe 141 over and over again. That rese¬ rvoir circuit and the standpipe 141 naturally keep the system covered with oil, and hence bubble free and cavita- tion free for*all such repeated manual brake applications.

BRAKE SET WITH FULL SPRING PRESSURE, UNDER POWER CONTROL — FIGURES 6, 8 Further movement of the connection 124 past the showing of Figure 5 by foot pressure of the operator causes the differential valve 16 to be pulled farther outwardly into the position as illustrated in Figure 6, opening up a space between the receding valve seat and the regulator valve 92. The idea is that the modulated release-pressure inlet-outlet line P^ reduces the pressure of differential chamber 60 down to drain pressure D, so that the springs 68 and 70 force the plunger 24 to braking position, under full spring pressure both unimpeded and unassisted. The graphical equivalents of the point reached by this pedal stroke, as illustrated in Figure 8, are the corresponding point 144 where the modulated pressure P^

becomes zero, and the corresponding point 146 where the braking torque P^ reaches a substantial amount. The reason the ball regulator valve 92 is off its seat is that the regulating spring 94 therefor is fully relaxed at the length illustrated in Figure 6, and so the inlet-outlet line Pi is freely ported at 132 to drain D. As already indicated, the short slots 102 are out of position except in full release condition and leave the retractor chamber 96 fully isolated from the inlet-outlet line P _j _. Finally, another set of short, longitudinal slots

148 on valve 16 which are of variable depth are illustrated stopping just short of the second bore core 86 so that none of the pressure P is allowed in the inlet-outlet line P2? in fact, tip to this point a third bore core 150 serving the line P2 has been continually connecting the main area chamber 62 through the variable depth slots 148 with the spring chamber 138 and drain line 140 leading to drain D. Therefore, so far, the unpressurized main pressure movable area 64 has neither aided nor interfered with operation of the plunger 24.

Further pull, however, on the valve 16 enables the variable depth slots 148 progressively to cut off from the spring chamber 138 and drain line 140 and progressively to open up the third bore core 86 which is supplied directly at line pressure P. So the modulated apply-pressure inlet- outlet line P2 is progressively pressurized through the slots 148 and third bore core 150 to affect the main pres¬ sure movable area 64 at the same time at which the differ¬ ential chamber 60 is thereafter always held at drain pres¬ sure by the modulated -release-pressure inlet-outlet line PI, with results now to be explained.

FULL POWER, BRAKE APPLIED — FIGURES 7, 8 Under this condition as illustrated in Figure 7, the differential piston 14 occupies the same four quarters or full stroke position as previously considered in Figure 1 and the chamber 96 under full line pressure P has its

maximum expansion. For the first time here considered, however, the valve 16 is shown pulled to maximum travel applying up to full line pressure to the main pressure movable area 64. In theory, the effective modulated pres- sure P2 in the modulated apply-pressure inlet-out line P2 will have reached the flat graphical line 152 in Figure 8 corresponding to line pressure P, whereas the brake torque T2 will have reached the full brake torque, flat graphical line 154. The stated values are believed rather excessive as a practical matter. In considering figures for comparative purposes only, by way of understanding and not of limita¬ tion, then in practice if we treat pressure P as 300 psi (2,050 kPa) , the modulated pressure P2 of value 200 psi (1,370 kPa) at point 156 in Figure 8 will afford a corres¬ ponding braking torque T2 at point 158 on the curve suf¬ ficient to meet just about all braking needs and effect a full stop under any condition.

It is a characteristic of power operation that a full valve stroke herein will provide an output pressure P^ decreasing fr<3m a substantial pressure level P at stroke initiation down to drain pressure D from and after mid¬ stroke 144, and a separate output pressure P2 increasing from drain pressure D, existing initially and including the point 144 of midstroke up to a substantial pressure level which is reached starting low and is fully reached at 152 (Figure 8) only at full stroke.

In Figure 7, release by the operator of the valve 16 from its illustrated position will allow the released spring to restore the brake system back into the full release condition of Figure 1.

From the foregoing, highly differentiated showings of the valve 16, it can readily be understood how both actuating chambers 60 and 62 of a master cylinder 36a are held by the valve 16 at drain pressure whenever required so as not to interfere with operater-operated unassisted or emergency assisted braking.

BRAKE-NEUTRAL PEDAL REDUNDANCY — FIGURE 1 When the brake pedal 126 is depressed, a torque converter 160 provided in the power shift transmission 28b enables the transmission to slip at its input side suf- ficiently to be overcome by the brakes. However, another member of the pedal assembly 30b known as a brake-neutral pedal 162 affords flexibility with the purpose that there can be no fight between the transmission 28b and the brakes 22 when the latter operate.

For this purpose, the header 84 supplies through a restricted line 164 line pressure to a normally closed transmission controlling valve 166 located in the assembly housing beneath the end cover 110 and having a plunger 168 projecting externally thereof. When hydraulic pressure can force the projecting plunger to extend outwardly into the broken line position 168a, the valve 166 connects line pressure P through a conduit 170 to a neutral valve N in the controls of the power shift transmission 28b.

Accordingly, when a connection 172 is operated between the pedal 162 and end 120 of the singletree, the pedal 162 in Seing depressed pulls the end 120 to swing to the broken line position 120a; the singletree fulcrum end 118 acts as fulcrum.

So with application of the brakes as caused by the outpulling of the valve 16, the projecting plunger 168 of the transmission controlling valve 166 takes the extended broken line position 168a sending out a pressure signal setting the transmission 28b automatically in neutral. The transmission drive therefore does not interfere with braking and no reliance need be placed on the torque con¬ verter 160 slipping to accommodate braking.

Secondarily so, because the brake-neutral ^• pedal 162 applies the brake independently of the brake pedal 126, a redundancy of action allows the operator to have a backup control in case of failure of the connection 124 or the brake pedal 126. The two pedals 126 and 162 share a common axis 173 of oscillation mutually defined by their horizon¬ tally disposed fixed pivots .

BRAKE PEDAL REDUNDANCY — FIGURE 1 Linkage failure will allow the brake pedal 126 to overtravel into the position 126a, confronting a normally closed valve 174. So the operator, by pushing the pedal 126 into overtravel, can under the circumstances open such normally closed brake apply valve 174 which directly inter¬ communicates the inlet-outlet line PI and the drain- connected standpipe 141 to fully depressurize the differ¬ ential area chamber 60. So, redundancy is inherently afforded to natural action of the operator allowing the springs 68 and 70 to apply the brakes in event of brake linkage failure.

HANDLE REDUNDANCY — FIGURE 9 In the modification as shown according to this illustration, a handle 176 provides hand backup for brake application. A sealed back cover 178 is supported by the assembly housing 12a and a pivot connection 180 on the cover supports the handle 176 for clockwise swinging, manual movement as viewed in Figure 9.

A dejsressible brake applying plunger 182 sealed at 184 to the back cover projects outwardly so as to be en¬ gaged in compression between the handle 176 and the differ¬ ential valve 16 with which it abuts. Downswing of the handle into the position as shown by the broken lines 176a depresses and causes the valve 16 to produce spring engagement of the brakes in the same way as described for the singletree 116. Release of the handle 176, releasing spring 186, allows release of the brakes because the valve 16 immediately resumes its position as shown by solid lines. What the spring 186 imparts to the valve 16 is a mechanical fail-safe operation.

The operator is thus afforded a redundancy by hand for setting the brakes. The ease with which the several re- dundancies stated can be afforded is made possible only by a mainspring 68 type of operator as herein described.

Careful analysis of Figure 8 foregoing will show that the modulated pressure PI is automatically pressure re¬ gulated by spring so as to have accurate predictable values and vary gradually at a constant, linear rate with spool stroke for smooth brake engagement initially. However, full brake application under control of the modulated pres¬ sure P2 need not be so sensitive and hence the pressure increase and decrease is rather abrupt per unit of spool stroke and not subject to automatic regulation by a pre- cisely spring—regulated valve.

Careful analysis of a converter driven pump 80 in normal operation will reveal that, at low speeds such as for low idle of the engine, the oil output can run only 2-3 gallons per minute and should best be devoted to control, solely of the transmission. In the present system, the brakes are applied not by taking away, from the trans¬ mission, output of pump 80 initially but by the immediately effective step of releasing pressure in the inlet-outlet line P^. So there is no chance of interrupting transmis- sion operation for even a fraction of a second by reason of sharing ^" its oil, and brake reaction to go into engagement is instantaneous because oil release from the chamber 60 is practically instantaneous .

In the so-called practice in the art termed a straight through braking operation under power failure, it is to be understood that the straight through braking hereof transpires each time the valve 16 is pedal operated to allow the ball 92 to unseat and drain the differential chamber 60. Then upon brake pedal release which follows sooner or later so as to allow the valve 16 to disengage the brakes, the check valve 143 ensures that the retractor chamber 96 retransfers all fluid to the differential chamber 60 and not return it into the standpipe reservoir system 140 or to the connections from the latter such as the drain D or standpipe 141 connected to the spring chamber occupied by the auxiliary spring 108.

At each point in return motion of the valve 16 from a projecting position, the retractor chamber 96 always stays under a residual positive pressure. So, the differ¬ ential area of the differential valve 16 serves as a pres- sure movable fail-safe braking area forcing the retracting valve to return to its brake disengaging position upon release of the brake pedal. In other words, the present differential valve design is not only mechanical fail-safe as noted at 186 but hydraulically fail-safe because of the residual chamber 96 pressure ensured from this unexpected function afforded by the auxiliary brake actuator spring 108. Of course with hydraulic failure at pump source 80, the oil will start to leak out after several minutes from the differential chamber 60 and the brakes will gradually apply; however, the engaged brakes can be quickly released again by stepping hard on either brake pedal 126 or 162 and then releasing it.

Because of the instantaneous response (as .compared with braking responses ordinarily relying upon pressure build-up) afforded under the principles hereof to release differential pressure and thus cause the fail-safe main spring immediately to apply the brakes, the present pedal- operated system and its dual utility for service braking and emergency braking offer significant operating advan- tages and at the same time are fully compatible with the companion small-design parking brake earlier suggested for the vehicle. The compatibility is evident upon stopping the vehicle on grade with the brake pedal, then setting the small parking brake, and releasing the brake pedal, because release of the pedal system transfers full braking load to the parking brake; so the operator can immediately sense whether or not the parking brake has taken hold. Then and thereafter because of inherent leakage past the differen¬ tial valve spool hereof, the gradual bleed-down of oil from the differential chamber, transpiring in the next couple of minutes or so, will let the re-expanding main spring in¬ herently reinforce the parking brake action by the indepen¬ dent braking action of hydraulically setting and holding the fail-safe pedal-system with brakes applied.