TM 9-2330-373-14 & P
Section III. PRINCIPLES OF OPERATION
1-12. BRAKE SYSTEM
When the air couplings are connected between the towing vehicle and the semitrailer, air shutoff valves on the towing
vehicle are opened. Air flows through the air lines and the relay valve into the air reservoir on the semitrailer. The air
pressure is built up to equal the air pressure on the towing vehicle.
When pressure is applied to the brake pedal on the towing vehicle, air pressure is directed to the relay valve.
The relay valve releases compressed air from the reservoir to the brake air chamber attached to the hydraulic master
The brake air chamber push rod extends to contact a piston inside the master cylinder. The piston is actuated by pressure
from the push rod to create hydraulic pressure in proportion to the pressure applied by the push rod.
The hydraulic pressure moves the wheel cylinder piston in the wheel brake mechanism. These pistons force the lining of
the brake shoe against the brake drum.
When the brake pedal is released a drop in pressure causes the relay valve to release the compressed air from the
semitrailer brake system.
With the air released, the brake return springs pull the brake shoes away from the drums.