Air- Over Hydraulic Cylinder

Wheeled Vehicle Braking Systems
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is like any typical hydraulic brake master cylinder and is used to

force fluid under pressure to the wheel cylinders in the wheel

brakes. The brake air chamber changes air pressure into mechanical

motion to operate the master cylinder. On tandemaxle trailers an

airoverhydraulic cylinder may be used on each axle.

The brake air chamber contains a diaphragm secured between the outer

edges of the body and cover. The diaphragm is airtight and divides

the chamber into pressure and nonpressure sides. The pressure side

of the chamber has a threaded opening for connecting the brake apply

air line. The nonpressure side is vented to the outside air. A

compression spring in the nonpressure side holds a pushrod against

the diaphragm, and returns both the diaphragm and pushrod to the

pressure side of the chamber. One end of the pushrod extends from

the nonpressure side of the chamber.

The master cylinder is mounted on the brake air chamber so that the

pushrod aligns with the master cylinder piston. When compressed air

enters the pressure side of the brake chamber, the diaphragm moves

toward the nonpressure side. This extends the pushrod, moving the

master cylinder piston, and forcing brake fluid to the wheel

cylinders to apply the brakes. When the compressed air in the

chamber is released, the compression spring returns the pushrod and

diaphragm to the pressure side of the chamber allowing the brakes to

release.