SPRING BRAKE ACTUATOR

A spring brake actuator (10) is for braking a wheel of a vehicle. The spring brake actuator (10) has an axially elongated housing (12) having a parking brake chamber (34) and a service brake chamber (36); a main compression spring (54) in the parking brake chamber (34); a flexible diaphragm (38) in the parking brake chamber (34); and a pressure plate in the parking brake chamber (34), the pressure plate (58) located axially between the main compression spring (54) and the flexible diaphragm (38). The flexible diaphragm (38) has a contoured portion (100) that inhibits radial shifting of the pressure plate (58) with respect to the flexible diaphragm (38).