Air pressure won't build
Can I keep driving?
The truck is parked until the air system is fixed. Stay with it, set the wheel chocks if you have them, and use the location box below to get ETAs from shops that come to you.
Do this first
- Listen for the leak with the engine running. Walk the truck and the trailer. A hiss you can hear from the ground is usually a line, a fitting, a glad hand or a brake chamber, and a mobile tech can fix all of those at the truck. Check the trailer glad hands first; a bad seal there is the single most common cause of air that will not build after a hookup.
- Watch the gauges, not the clock. Note whether the needle climbs slowly and stalls, climbs to a point and then drops, or never moves at all. Slow and stalling points at a big leak or a weak compressor. Climbs then drops with the engine off is a leak. Never moves is the compressor, the governor or the unloader.
- Check the air dryer and the tanks. Pull the drain valves. A tank full of water or oil, or a dryer that purges constantly, tells the tech what to bring. In freezing weather, ice in the lines or the dryer is the first suspect, and that one you can sometimes thaw with time and heat, not with alcohol poured into the system.
What it usually costs and how long it takes
Air-system work is cheaper than it sounds and faster than a tow:
- Line, fitting or glad-hand seal. The cheapest outcome. A mobile tech carries the fittings and the seals; this is often a thirty-minute roadside job once they arrive.
- Brake chamber or diaphragm. A roadside replacement on most axles. The part needs to match the chamber size.
- Air dryer cartridge or purge valve. Roadside on many trucks; a dryer that has been neglected can also mean contaminated tanks that need draining.
- Compressor, governor or unloader valve. The expensive end. Governors and unloaders are small parts; a compressor is a shop job and sometimes a tow.
The time you lose is mostly waiting for the tech, which is why this page sorts by who can reach you.
Where to get it fixed
Cities with the most shops listing brake and air-system work:
Frequently Asked Questions
The air builds, but it takes forever. Is that the same problem?
It is the early version of it. Slow build with no audible leak usually means a tired compressor or a dryer that is purging too often. It will keep getting slower. Get it looked at before it becomes a cannot-move morning.
Can I cage the brakes and drive to the shop?
No. Caging releases the spring brakes mechanically, which lets the truck roll, but with no air there are no service brakes either. Caging is for moving a disabled truck a few feet onto a tow truck or out of a lane with a spotter, not for driving.
It was fine before I hooked the trailer.
Then the leak is on the trailer side or at the glad hands. Disconnect the trailer lines, and if the tractor builds and holds air on its own, you have found the side of the problem. The tech still needs to come, but now they know where to look.
Does cold weather cause this?
Yes, often. Moisture in the lines freezes at the lowest point, usually a valve or a low loop in a line. A dryer with a spent cartridge lets that moisture through. Thawing with careful heat solves today; replacing the cartridge solves the winter.
What is the air governor and why do techs ask about it?
The governor tells the compressor when to pump and when to rest. If it sticks in the rest position, the compressor never pumps and the gauges never move even though nothing is leaking. It is a small, cheap part that mimics a dead compressor.
How much pressure should the truck hold?
Operating range on most trucks is roughly 100 to 130 psi, with a low-pressure warning that comes on well before the spring brakes apply. Your dash and your pre-trip tell you the exact numbers for your truck; if the warning is on, the truck is not ready to move.
- The air builds, but it takes forever. Is that the same problem?
- It is the early version of it. Slow build with no audible leak usually means a tired compressor or a dryer that is purging too often. It will keep getting slower. Get it looked at before it becomes a cannot-move morning.
- Can I cage the brakes and drive to the shop?
- No. Caging releases the spring brakes mechanically, which lets the truck roll, but with no air there are no service brakes either. Caging is for moving a disabled truck a few feet onto a tow truck or out of a lane with a spotter, not for driving.
- It was fine before I hooked the trailer.
- Then the leak is on the trailer side or at the glad hands. Disconnect the trailer lines, and if the tractor builds and holds air on its own, you have found the side of the problem. The tech still needs to come, but now they know where to look.
- Does cold weather cause this?
- Yes, often. Moisture in the lines freezes at the lowest point, usually a valve or a low loop in a line. A dryer with a spent cartridge lets that moisture through. Thawing with careful heat solves today; replacing the cartridge solves the winter.
- What is the air governor and why do techs ask about it?
- The governor tells the compressor when to pump and when to rest. If it sticks in the rest position, the compressor never pumps and the gauges never move even though nothing is leaking. It is a small, cheap part that mimics a dead compressor.
- How much pressure should the truck hold?
- Operating range on most trucks is roughly 100 to 130 psi, with a low-pressure warning that comes on well before the spring brakes apply. Your dash and your pre-trip tell you the exact numbers for your truck; if the warning is on, the truck is not ready to move.
This page is general guidance for professional drivers and is not a diagnosis of your truck. Brake and air-system work is safety-critical; never move a truck whose brakes have not been restored and tested. Shop listings are provided by Trucker Path users and the businesses themselves; availability and pricing are confirmed by the shop, not by Trucker Path.