Parking Brake System
Spring brakes, holding power, and why you never use parking brakes as service brakes.
Endorsement: Air Brakes · Source: FMCSA CDL Manual (public domain)
The parking brake on an air-braked vehicle is mechanically applied by powerful coil springs inside the spring-brake chambers at each rear wheel. To release the parking brake, the driver pushes in the yellow diamond-shaped knob on the dashboard, which directs air into the spring-brake chambers and compresses the springs. To set the parking brake, the driver pulls the knob out, which exhausts air from the chambers and lets the springs apply the brakes.
Because the spring brakes are applied mechanically, they will hold the vehicle even with no air in the system. This is why a parked truck with a slow leak will not roll away — as pressure drops, the springs apply the brakes more firmly, not less. It is also why you cannot use the parking brake as a regular service brake: applying it at speed locks the rear wheels suddenly and can cause a jackknife. The only legitimate use of the parking brake at speed is as a last-resort backup if service brakes have completely failed, and even then it should be applied gently if any service-brake authority remains.
A combination vehicle adds a separate trailer parking brake system. On a typical tractor-trailer, the red octagonal trailer-air-supply knob (sometimes labeled "trailer brakes") controls the trailer parking brakes. Pulling out the red knob disconnects the trailer from the tractor air supply and applies the trailer spring brakes. Pushing in the red knob charges the trailer air system and releases the trailer brakes; you must do this before attempting to move. Combination drivers also use the hand valve (sometimes called the trolley valve) to apply only the trailer service brakes — useful for testing the trailer brake connection but not for driving, because using the hand valve to slow the vehicle invites trailer jackknife.
Key terms to memorize
- compressor
- governor
- supply tank
- service tank
- brake chamber
- slack adjuster
- low-pressure warning
- spring brake
- application gauge
Other Air Brakes topics
- Air Brake System Components — How an air brake system actually moves a brake shoe — every part, in order.
- Seven-Step Pre-Trip Air Brake Test — The mandatory pre-trip sequence that proves your brakes will work when you need them.
- Dual Air Systems — Why modern trucks have two independent air brake systems and how they protect you.
- Low-Pressure Warning Devices — How the system tells you that braking authority is about to be gone — and what to do.
Test what you learned
Now that you have the Parking Brake System material in your head, drill the Air Brakes practice test. The questions are drawn from the same FMCSA source material this article paraphrases. For state-specific framing, jump to your state page and pick the Air Brakes test for your jurisdiction.