PURPOSE: To ensure accurate return movement of a second member regardless of expansion of the hydraulic fluid in a system where the movement of a first member is translated through fluid in a fluid chamber to movement of the second member by providing a pressure equalization line which communicates the fluid chamber only when the second member is near a first position.

CONSTITUTION: This device contained within a body has an electronic expansion module like a piezoelectric motor 10, a first housing 12 screwed on a bore of the body, and a second housing 14. The device contains a larger diameter piston (the first member) 16 in the second housing 14 adjoining the piezoelectric motor 10, and a smaller diameter piston (the second member) 20 in the same second housing 14. A fluid chamber 22 is defined between the two pistons 16, 20, and a pressure equalization line 30 is provided such that it communicates with the fluid chamber 22 through an annular groove 32 around the smaller diameter piston 20. Fluid flow into the fluid chamber 22 is allowed only when the piston 20 is in a first position as illustrated.