A spring brake actuator for applying a brake of a vehicle, the spring brake actuator comprising: a housing containing a diaphragm that separates the housing into first and second chambers (21, 22); a push rod assembly having a push rod extending out of the second chamber; wherein the diaphragm is flexible in a first direction to retract the push rod inwardly relative to the second chamber and in an opposite second direction to extend the push rod outwardly from the second chamber; a compression spring in the first chamber; a return spring in the second chamber; a port for conveying pressurized air to the first chamber; and a clutch actuator device for selectively compressing the compression spring, wherein the spring brake actuator is operable in a plurality of states comprising: a parking state in which the clutch actuator device permits extension of the compression spring, which flexes the diaphragm in the second direction, compresses the return spring, and extends the push rod further outwardly from the second chamber for applying the brake of the vehicle; a driving state in which the clutch actuator device compresses the compression spring, which permits the return spring to extend, which flexes the diaphragm in the first direction and retracts the push rod further inwardly relative to the second chamber for disengaging the brake of the vehicle; and a braking state in which the clutch actuator device compresses the compression spring, and further in which pressurized air is conveyed to the first chamber via the port, which flexes the diaphragm in the second direction, which compresses the return spring and extends the push rod further outwardly from the second chamber for applying the brake of the vehicle. a housing containing a diaphragm that separates the housing into first and second chambers (21, 22); a push rod assembly having a push rod extending out of the second chamber; wherein the diaphragm is flexible in a first direction to retract the push rod inwardly relative to the second chamber and in an opposite second direction to extend the push rod outwardly from the second chamber; a compression spring in the first chamber; a return spring in the second chamber; a port for conveying pressurized air to the first chamber; and a clutch actuator device for selectively compressing the compression spring, wherein the spring brake actuator is operable in a plurality of states comprising: a parking state in which the clutch actuator device permits extension of the compression spring, which flexes the diaphragm in the second direction, compresses the return spring, and extends the push rod further outwardly from the second chamber for applying the brake of the vehicle; a driving state in which the clutch actuator device compresses the compression spring, which permits the return spring to extend, which flexes the diaphragm in the first direction and retracts the push rod further inwardly relative to the second chamber for disengaging the brake of the vehicle; and a braking state in which the clutch actuator device compresses the compression spring, and further in which pressurized air is conveyed to the first chamber via the port, which flexes the diaphragm in the second direction, which compresses the return spring and extends the push rod further outwardly from the second chamber for applying the brake of the vehicle. a parking state in which the clutch actuator device permits extension of the compression spring, which flexes the diaphragm in the second direction, compresses the return spring, and extends the push rod further outwardly from the second chamber for applying the brake of the vehicle; a driving state in which the clutch actuator device compresses the compression spring, which permits the return spring to extend, which flexes the diaphragm in the first direction and retracts the push rod further inwardly relative to the second chamber for disengaging the brake of the vehicle; and a braking state in which the clutch actuator device compresses the compression spring, and further in which pressurized air is conveyed to the first chamber via the port, which flexes the diaphragm in the second direction, which compresses the return spring and extends the push rod further outwardly from the second chamber for applying the brake of the vehicle.