The principle of operation of air brakes in a trailer.
The trailer brakes are operated as follows: When the brake pedal of the towing vehicle is depressed, the brake valve also opens, the trailer control system and deflates the line connecting the vehicle to the trailer. The trailer auxiliary valve is then relieved of the load, moves and deflates the air from the trailer reservoir to the trailer brake cylinders. The pistons in the cylinders turn the spreaders as they move, which in turn press the shoes against the brake drums. The trailer is braked together with the towed vehicle. To unbrake the trailer, the towing vehicle's brake pedal must be released; the pressurized air then passes into the line connecting the towed vehicle with the trailer and acts on the auxiliary valve of the trailer's tank. This valve closes the air supply from the trailer's tank and at the same time opens the air outlet from the trailer brake cylinders. The brake is disinhibited.