Your truck will not build air. Is it the compressor, or something that only looks like it?

How the air system is supposed to build

The compressor on a heavy truck is driven by the engine and runs whenever the engine does. It pumps air through a discharge line to the air dryer, which strips out moisture, and on into the supply tank and the brake system's tanks. A governor watches the pressure. When the system reaches its cut-out pressure — on most trucks somewhere around 120 to 135 psi — the governor signals the compressor to unload, so it keeps turning but stops pumping. When pressure falls back to the cut-in point, typically about 20 psi lower, it starts pumping again, and the air dryer purges the moisture it has collected.

A healthy system builds quickly. A standard check drivers learn is that on a dual air system, pressure should climb from 85 to 100 psi within about 45 seconds at normal operating engine speed. A system that is much slower than that, or that stalls below its normal cut-out pressure, has a problem somewhere in that chain, and the gauge alone does not say where.

Why it matters right now

A truck whose air system will not build is a truck whose brakes are running out of margin. The low-air warning comes on before pressure falls to 60 psi, and as pressure keeps dropping, the tractor protection valve closes and the spring brakes begin to apply, typically somewhere between 20 and 45 psi. On a loaded truck at highway speed, that sequence is not something to discover in the fast lane.

If the low-air warning comes on while you are driving, get off the road as soon as it is safe to do so, rather than trying to make the next exit that suits you. If the truck will not build air before you set off, do not set off. Our post on spring brakes that will not release covers what happens at the far end of that sequence.

What is usually behind it

• A governor that is stuck or faulty. If the governor keeps the compressor unloaded, it turns without pumping, and pressure never builds. A governor whose signal line has frozen or become blocked can do the same.
• An unloader that is stuck open. The unloader is what actually stops the compressor pumping when the governor tells it to. Stuck in the unloaded position, it leaves the compressor running but useless.
• An air dryer purging all the time. If the dryer's purge valve sticks open, air blows straight out of the dryer as fast as the compressor delivers it. A loud, continuous rush of air from the dryer is the giveaway.
• A blocked discharge line. The line from the compressor to the dryer carries hot, moist air. In freezing weather water can freeze in it, especially at low points; over time it can also narrow with carbon. Either way the compressor pumps against a restriction and the system builds slowly or not at all.
• A leak bigger than the compressor's output. A split line, a blown chamber diaphragm or a failed fitting can lose air as fast as it is made. The gauge looks like a compressor fault; the hiss says otherwise. Our post on air leaks: roadside repair or tow covers finding them.
• A worn compressor. Worn valves and rings pump less air and pass more oil. Slow build-up that has been getting gradually worse, oil appearing where the tanks drain, and an air dryer cartridge that fails early are the usual signs.
• A restricted intake. On many trucks the compressor draws its air through the engine's intake system, so a badly restricted air filter can reduce what it has to pump.

What a driver can check safely

Do not keep driving a truck whose air pressure is falling or will not build, hoping it recovers. Once it is parked somewhere safe with the wheels chocked, the checks below help narrow the cause down. Note what you find, including the build time, where the gauge stopped, whether the dryer was purging and what came out of the tank drains, and give those details when you call. They often let a technician identify the likely fault before the service truck leaves, so the right governor, dryer parts or line fittings come on the first trip.

• Listen at the air dryer. A continuous blast of air from the purge valve while the system is trying to build points straight at the dryer or the governor.
• Listen for leaks elsewhere. Walk the truck with the engine running and the brakes released, listening along the lines, at each wheel and at the glad hands.
• Time the build. Note how long the gauge takes to climb and where it stops. A system that reaches cut-out slowly is a different problem from one that stalls halfway.
• Check the tank drains. Water is normal in small amounts and points at the dryer when there is a lot; oil points toward the compressor.
• Think about the temperature. A truck that built air fine yesterday and will not on a hard frozen morning very often has ice somewhere, rather than a dead compressor.

What gets fixed where the truck stands

Many of the causes above are roadside repairs. Governors, air dryer purge valves and dryer cartridges, air lines, fittings and the leaks that drain the system are replaced or repaired on site by the service truck. A frozen discharge line or governor signal line can be cleared carefully, without an open flame, and the moisture problem behind it traced so it does not return the next cold night.

Once the repair is done, the system is built back up to cut-out, the build time is checked, the governor is confirmed to cut in and out properly and the system is checked for leaks before the truck moves. The mobile truck repair page lists the faults the service truck handles most often.

When it is shop work

Replacing the compressor itself is usually not a roadside job. On most heavy trucks it is mounted on and driven by the engine, and changing it properly is a substantial repair better done in a shop with the right access and the ability to deal with anything else that turns up once it is apart. The same is true when a failing compressor has been pumping oil into the air system long enough to contaminate valves downstream.

When that is the answer, the useful thing is to know it before the truck is stranded somewhere worse. A truck that will not build air cannot be driven to a shop, and it should not be towed with its spring brakes dragging. A heavy wrecker can supply air to release the brakes through the glad hands, or lift the axles clear, and take the truck where it needs to go. In winter, read gelled fuel and frozen air lines as well, because moisture in the air system is behind a large share of cold-weather air problems.

Related reading

Mobile Diesel Mechanic: What They Fix, and What They Don't — A mobile diesel mechanic is a technician who drives to your truck with the diagnostic gear, the tools and the common parts, instead of the truck being towed to a shop.

Semi Brakes Won't Release: What to Check First — Spring brakes are held off by air, so brakes that will not release almost always mean air is not reaching them.

Semi Running Hot: Pull Over or Push On to the Next Exit? — A climbing gauge under heavy load on a grade can be normal; a gauge that keeps climbing after you shed load and speed is not.