Parking Brake Won't Release: Why a Truck or Trailer Stays Locked

The direct answer: spring brakes are held off by air pressure. If the parking brakes will not release, either air is not reaching the spring chambers, or the brakes are mechanically stuck even though the air is there. Low system pressure, a control valve that was not pushed in, crossed or leaking gladhands, ice in the lines and shoes frozen to the drums cover most cases. Do not drive against a brake that has not fully released, and never take a spring brake chamber apart.

First: can you keep driving?

No. A brake that is still applied or dragging turns the truck’s power into heat at that wheel: glazed linings, cracked drums, cooked wheel seals and, at the far end, a wheel-end fire. Stop and call if the truck or trailer will not roll freely once pressure is up, if one wheel stays locked, or if the pressure will not build at all. (916) 571-8888.

Work through it in this order

1. Is the air up? Let the system build to its normal operating pressure before judging anything. Spring brakes stay applied on a system that is still low.
2. Are both dash valves in? The yellow parking valve and, with a trailer, the red trailer air supply valve. A red valve that pops back out is telling you the trailer side is losing air.
3. Gladhands. Seated, seals present, red to emergency and blue to service. Crossed lines keep the trailer’s spring brakes on.
4. Listen for a leak with the valves pushed in. A steady hiss at a chamber, a valve or a hose is the answer. See air pressure loss.
5. Which wheels are held? Every wheel on the trailer points to supply air; one wheel points to that brake.

What causes it

• Low or lost air — a leak large enough that pressure never reaches the level that holds the springs off.
• Ice in the air system. Water that should have been removed by the air dryer freezes in a line or valve and blocks the air that releases the brakes. Common on the first cold mornings and on Sierra runs.
• Shoes frozen to the drums. Parking with the brakes set while they are wet lets the linings freeze to the drum in sub-freezing weather. The chamber releases and the shoes do not.
• A failed spring brake chamber — a ruptured diaphragm leaks the release air away, or a broken power spring jams the chamber.
• Valves — a sticking relay valve, quick-release valve, tractor protection valve or dash control valve.
• Seized foundation brake parts — an S-cam, slack adjuster or caliper that does not return. More in trailer brakes locking or dragging.
• A caged brake left caged, or the opposite — a release bolt partly wound in or out after earlier work.

What not to do

• Do not open a spring brake chamber. The power spring inside is compressed with enough force to kill. Chambers are replaced as units.
• Do not drag it free with the engine. Pulling a locked wheel flat-spots the tire and can tear the lining from the shoe.
• Do not cage a brake and carry on with the trip. Winding in the release bolt removes the parking and emergency brake from that wheel. It is a way to move a disabled vehicle for repair, not a repair.
• Do not go under a truck that is not chocked. Once a stuck brake lets go, the vehicle can roll.

How we fix it

We find where the release air stops or which brake is holding, then repair that: the leaking chamber or hose, the valve, the seized cam or adjuster, or the moisture at its source in the air dryer. A brake that dragged for any distance gets its lining, drum and wheel seal checked before the truck goes back out. See brake inspection and repair; many of these are fixed where the truck sits through mobile truck repair.

Common questions

Why are my trailer brakes locked after it sat overnight in the cold?

Either moisture froze in the air lines or valves, or the shoes froze to the drums because the brakes were set wet. Build full air first. If the chambers release and a wheel still will not turn, it is the shoes.

How do I keep brakes from freezing?

Drain the air tanks, keep the air dryer serviced, and after driving through water or slush, dry the brakes with light applications before parking. Our winter prep guide covers the air system.

The red trailer supply valve keeps popping out. Why?

It is doing its job: it closes when trailer supply pressure falls, to protect the tractor’s air. Look for a leak on the trailer, a gladhand seal or a ruptured chamber.

Does the parking brake have to work for an inspection?

Yes. Federal rules require a parking brake system able to hold the vehicle or combination stationary under any condition of loading (49 CFR 393.41), and it is checked at the annual inspection.