Hydropneumatic brake actuator arranged to maintain a constant brake shoe clearance

A vehicle brake actuator device for use with a vehicle having a friction brake surface and brake means movable into and out of engagement therewith, said brake actuator device comprising:

• (a) a cylinder body having a bore therein with an open end and a closed end;
• (b) a power piston operably-disposed in said bore and forming at its one end in cooperation with said closed end of said bore a chamber in which the presence of fluid pressure is effective to actuate said power piston from a brake release position to a brake application position;
• (c) means for urging movement of said power piston toward said release position;
• (d) friction means for providing a stop with which said power piston is engageable to establish said release position thereof in the absence of fluid pressure in said chamber, comprising:
• (i) first and second pressure plates in said chamber between said power piston and said closed end of said bore;
• (ii) an annular groove formed jointly by said first and second pressure plates at the periphery thereof;
• (iii) elastomeric means in said annular groove having frictional engagement with said bore;
• and
• (iv) means for adjusting the relative axial position of said first and second pressure plates to vary the width of said groove and thereby the radial pressure with which said elastomeric means engages said bore;
• and
• (e) means for providing a lost-motion connection between said power piston and said friction means via which said friction means is axially displaced along said bore subsequent to movement of said power piston a predetermined distance from said release position, said predetermined distance corresponding to the distance said brake means is retracted from said braking surface during movement of said power piston from said brake application position to said brake release position.