Abstract: A spring brake actuator assembly for actuating a brake includes a spring chamber and a service chamber. A central flange defines opposing first and second cup-shaped sections. A spring chamber housing is interconnected to the first cup-shaped section of the central flange case for defining a spring chamber. A reciprocating piston is disposed in the spring chamber and includes a tubular push rod being biased into the service chamber from the spring chamber by a power spring disposed between the spring chamber housing and the reciprocating piston. The spring chamber is sealably bifurcated into a spring portion and a brake release portion. The brake release portion is subject to pneumatic pressure for compressing the power spring disposed in the spring chamber. A service flexible diaphragm defines the service chamber that is also subject to pneumatic pressure for driving a push rod outwardly of the service chamber to actuate a brake.