Abstract: A piston and cylinder assembly for a vehicle hydraulic system has a retaining device comprising an insert acting between a piston and the cylinder bore in which it works, the insert having a releasable frictional engagement in the bore and acting as an abutment to hold the piston in its retracted position to maintain the effective length of the assembly at a predetermined value for installation, until the assembly is operated hydraulically. The assembly may be an actuator for a self-energizing disc brake, or a slave cylinder for a vehicle clutch system.