The brake system on a truck requires high levels of air pressure to stop and begin moving again. Long-haul trips often require the driver to pump the brakes many times over for hours at a time. This can cause the brakes to lose their high-pressure state and they may start to fail. Trucks equipped with spring brakes provide a back-up to ensure the driver can come to a controlled stop if their service brakes are unable to engage.

The spring brake cylinder combines the service and parking brake chambers in one unit that is mounted between the chassis and axles. In order to operate, the spring brake cylinder needs to have the same air pressure as the service brake chamber. Air passes through the triple valve system and air lines to reach the brake cylinder. Air pressure is built up in the brake cylinder until it is strong enough to push a rod (known as a pushrod) into one end of the cylinder. The other end of the cylinder is sealed with a piston and has two springs that apply force against it to keep it in place.

When the driver presses the brake pedal, air is forced into one of the spring brake cylinder pistons and it moves a bridge that connects to a brake pad on each wheel. The pressure from the bridge forces the brake pads against the rotor to create friction and slow the wheel’s rotation. When the operator releases the brake pedal, the return springs push the caliper and bridge back to their original positions. The caliper is then released to allow the wheel to spin freely again.

The double-parking control valve, located on the dash board of a vehicle, has two push-pull knobs. The left-hand knob is used to put on the parking brakes and the right-hand knob is used to release the parking brakes. Both of these knobs are pushed in to supply air and pulled out to exhaust the air to activate the parking brakes.

Most trucks with dual park brakes have a separate air tank that can be used to release the spring brakes in the event the main air pressure fails. Larger vehicles, such as buses, will have a switch on the dash board that will let you manually activate the parking brakes using this independent air source.

Modulating control valves on a truck can also be used to reduce the speed at which the spring brakes come on when you are driving. These are normally push-pull type valves that have a knob on the dash board that is pushed to move air in and out of the brake cylinder.