Your truck's parking brakes won't release. Here is what is usually holding them.

Why spring brakes stay on

The parking brakes on a tractor and trailer are spring brakes. Inside each spring brake chamber is a powerful coil spring that applies the brake on its own, and the only thing that holds that spring back is air pressure. Push the parking brake control in and air flows into the chambers, compresses the springs and releases the brakes. Lose the air and the springs apply them again.

That design is deliberate. A truck that loses air pressure stops rather than rolls, which is exactly what you want from a braking system. But it also means that when brakes will not release, the question is almost always the same one: why is air not reaching the chambers, or why is it escaping from them? On most trucks the controls are the yellow, diamond-shaped parking brake knob and the red, eight-sided trailer air supply knob, and both depend on the system having air to send.

Check these first, before you call

Some causes are found in a couple of minutes from the cab and the catwalk. Work through these in order, and stop the moment anything looks unsafe.

• Chock the wheels. Before anyone touches brakes that might suddenly release, put chocks in front of and behind a wheel. A truck parked on even a slight grade can roll the moment the brakes let go.
• Build full air. Run the engine and watch both air gauges climb until the governor cuts out, which on most trucks is somewhere around 120 to 135 psi. The parking brake knob will not stay in if pressure is too low, and a system that will not build is a different problem from one that builds but still will not release.
• Push the red trailer air supply knob in. If it will not stay in, or pops back out, air is being lost on the trailer supply side, often through a disconnected, damaged or leaking line.
• Check the glad hands. The emergency line, usually red, must connect to the trailer's emergency glad hand, and the service line, usually blue, to the service side. Crossed lines are one of the most common reasons trailer brakes will not release right after hooking up.
• Listen for air. A steady hiss near one wheel while the brakes are supposed to be released points straight at the chamber or the fittings on that wheel.
• Think about the weather and how long it sat. Moisture frozen in the lines or valves after a cold night, or brake shoes rusted to the drums after a truck has stood for weeks, both hold brakes on even with good air.

What is usually behind it

When the brakes will not release at all, the cause is usually one of the following. Brakes that release but then drag, or a single wheel that heats up once the truck is moving, are a related but different problem. Our post on a trailer brake dragging or a hot hub covers that one.

• Low system pressure. A compressor that is not keeping up, a governor fault or a large leak means the system never reaches the pressure the spring brakes need.
• Trailer supply problems. Crossed or loose glad hands, a split supply line or a torn glad hand seal starve the trailer's spring brakes of air.
• A failed spring brake chamber. When the diaphragm on the parking side of a chamber tears, air escapes as fast as it arrives and that wheel's brake stays applied.
• A sticking or failed valve. The parking brake control, relay valves and quick-release valves all route air to the chambers; one that sticks can hold brakes on even with a full system.
• Ice. Water in the air system freezes in lines and valves in cold weather and blocks air from reaching the chambers. An air dryer that is not doing its job makes this far more likely.
• Shoes stuck to drums. After a long park, or a wet day followed by a hard freeze, shoes can stick to the drums even once the chambers have released.

Tractor or trailer? Look at which wheels are holding

Which wheels stay locked narrows the search quickly. If the tractor's drive axles release but the trailer's brakes stay on, the fault is on the trailer side: the supply line, the glad hands, the trailer's own valves or a trailer chamber. If the tractor's brakes will not release either, start with the tractor's air supply, its parking brake control and its own chambers.

Tell dispatch which it is when you call. It decides whether the service truck brings parts for the tractor, the trailer or both, and it saves a trip back for something that could have been on board.

What gets fixed where the truck stands

Most brakes that will not release are a roadside repair. Spring brake chambers, air lines, fittings, glad hands and seals, and many of the valves that route air to the chambers are replaced on site by the service truck. Frozen lines can be cleared, carefully and without an open flame, and the moisture problem that caused them can be traced to the air dryer so it does not happen again the next cold night.

Once the repair is done, the system is aired up, checked for leaks and the brakes are confirmed to apply and release properly before the truck moves. The mobile truck repair and trailer repair pages list what the service truck handles most often.

Caging the brakes: when it helps and when it does not

A spring brake can be released mechanically by caging it: turning the release bolt built into the chamber compresses the spring by hand, so that wheel's brake lets go without air. It is a real tool, and it is how a truck with a failed chamber is moved off a live lane or prepared for a tow.

It is not a fix, and it should only be done by someone who knows how. A caged wheel has no parking brake and no emergency braking, so the wheels must be chocked before the bolt is turned, and the vehicle should only move as far as it has to — to safety, to a tow or into a repair. Driving any distance on caged brakes is dangerous, and brake defects like this are exactly what puts a truck out of service at an inspection; see DOT out-of-service faults for that side of it.

When it becomes a tow

Some causes are not roadside repairs. A compressor that has failed, a major valve that cannot be sourced quickly, or drums and shoes that are damaged rather than simply stuck are shop work. So is any case where the brakes turn out to be the symptom of a bigger air system failure.

When the truck has to move and its brakes will not release, a heavy wrecker can supply air to release the brakes through the glad hands, or lift the locked axle clear of the ground, so the truck is towed without dragging locked wheels along the road. If the truck is stuck in a live lane, that move comes first and the diagnosis comes after. In cold weather, our post on gelled fuel and frozen air lines covers how to keep this from happening at all.

Related reading

What Mobile Truck Repair Costs in Kansas City, and Why — Mobile truck repair is priced per job rather than from a flat rate card, because the bill depends on the actual fault, the parts, how long diagnosis and repair take, and where the truck is sitting.

Where to Place Warning Triangles When a Semi Breaks Down — Turn on your hazard flashers immediately, then place three warning triangles as soon as possible and within 10 minutes.

Can a Trailer Be Towed Without Its Tractor? — Yes.