Parking Brakes and Emergency/Spring Brake Systems
How a spring brake works
A spring brake chamber contains a large mechanical spring that is held compressed by air pressure. When you release the parking brake, air pushes and holds the spring back. When air pressure is lost - deliberately through the parking brake control, or because of a failure - the spring extends and applies the brake. This is the key safety feature: loss of air means the brake comes on, not off.
The control is normally the yellow diamond push-pull knob. Pulling it exhausts air from the spring chambers so the brakes set; pushing it in charges them so the brakes release. On a combination vehicle, the tractor protection valve and the trailer supply valve (red) protect the tractor's air if the trailer separates.
Caging and anti-compounding
A caging bolt lets you mechanically compress and hold a spring so a vehicle can be moved for repair - it is not a normal driving mode. An anti-compounding feature prevents service brake application from adding to spring force and overloading the chamber.
Traps
- Spring brakes are not 'either on or off' at zero pressure - they apply progressively as pressure falls through the 20–45 psi band.
- You cannot simply drive away with low pressure: the spring brakes will drag or hold.
- Never disassemble a spring brake chamber - the spring stores lethal energy.
- The parking brake alone is not a substitute for chocking wheels on a grade.
Rule of thumb: if a spring brake will not release, the problem is usually air supply to that chamber - check the relay valve, hose and the parking control before touching the chamber.