a cylindrical housing having a central, axially extending longitudinal axis;

a single diaphragm within the housing, the diaphragm dividing the housing into front and rear variable-volume chambers;

a push rod axially disposed along the central axis of the housing, the push rod having a proximate end and an opposite distal end, the proximate end being disposed in the front chamber and terminating in a top plate that is engaged with the diaphragm, the distal end extending outwardly from the housing;

a parking spring axially disposed in the rear chamber along the central axis of the housing, the parking spring urging the diaphragm against the top plate in a direction towards the front chamber;

a valve body extending axially through the rear chamber along the central axis of the housing, the valve body including an elongated stem coaxially aligned with the push rod, the stem extending into an opening in the top plate and being disposed in a telescoping relationship with the push rod;

a first supply passage in fluid communication with a valve port in the valve body;

the valve body including a passageway through the stem in fluid communication with the valve port, and the valve body further including a plurality of open and closeable transfer ports in fluid communication with the rear chamber;

the push rod including an internal chamber in fluid communication with the passageway in the valve body, the push rod further including a plurality of front chamber ports, the internal chamber of the push rod being in fluid communication with the front chamber via the plurality of front chamber ports; and

a second supply passage in fluid communication with the rear chamber via a plurality of rear chamber ports;

wherein in a resting mode with no pneumatic pressure in the front and rear chambers, brake pressure is applied by the parking spring to the top plate, in a deactivation mode pneumatic pressure is supplied to the front chamber to compress the parking spring and disengage the brake, and in an activation mode pneumatic pressure is transferred from the front chamber to the rear chamber, and pneumatic pressure is also supplied to the rear chamber via the second supply passage, to apply the brake function with both the pneumatic pressure and a spring force of the parking spring acting on the top plate.