Parking Brakes and Spring Brake Operation
How spring brakes work
Spring brakes are the fail-safe parking brake on air-braked vehicles. A strong spring pushes the brake into the applied position; air pressure pushes against the spring to release the brake. That design means losing air pressure sets the brakes - the opposite of a hydraulic parking brake.
Key facts
- Parking brake control - a yellow, diamond-shaped push-pull knob. Pull it out to apply the spring brakes; push it in to release them (once air pressure is sufficient).
- Release requires air. If pressure is too low, the knob will not stay in or the brakes will not release. Build pressure first.
- Anti-compounding - on vehicles with this feature, the service brake and spring brake do not add their forces together, protecting the mechanism.
- Modulated spring brakes - some vehicles let you feather the spring brakes with the control for a controlled stop, but they are not a substitute for service brakes.
- Trailer spring brakes - the trailer supply line (emergency line) charges the trailer; if it separates, the trailer spring brakes apply automatically.
Traps
Questions often ask what happens when air pressure is lost while parked - the answer is that the spring brakes stay applied, which is exactly what you want. Another trap is assuming you can release spring brakes with no air; you cannot without a mechanical release (caging) and that is only for moving a disabled vehicle, not for normal driving.
Worked example
A driver shuts down with the parking brake set and hears air escaping. Pressure falls and the knob stays out. That is normal - the spring brakes are holding. If the driver later cannot push the knob in, the cause is insufficient air pressure, not a broken knob.
Rule of thumb
"Air releases, springs apply." Whenever a question asks what happens without air, the answer is that the brakes set.