An improved diaphragm-type pneumatic brake actuator wherein the diaphragm (92) is releasably secured to the piston (102) as the diaphragm is inverted during actuation of the braking system of a vehicle thereby reducing wear of the flexible diaphragm (92) and improving the performance of the brake actuator. In the disclosed embodiment, a rib or lip (110) is formed on the surface of the diaphragm which receives the periphery of the piston as the diaphragm (92) is inverted forming a mechanical interlock between the diaphragm (92) and the piston (102) thereby preventing spacing of the contact surface of the piston b(102) and the central portion of the diaphragm (92) and frictional wear between the peripheral edge of the piston and the diaphragm. In the preferred embodiment, the piston includes an arcuate rim (108) which extends away from the central portion of the diaphragm (92) when the diaphragm is located in its extended cup-shaped position prior to actuation of the vehicle braking system and the diaphragm then conforms to the arcuate rim (108) as the diaphragm (92) is inverted and the diaphragm rib (110) snaps over the piston rim as the diaphragm (92) is inverted to actuate the vehicle braking system. The diaphragm rib (110) may be molded on the surface of the diaphragm when the diaphragm is molded without any substantial increase in cost.
