Why air brakes deserve extra attention
Brakes are the top vehicle out-of-service category at CVSA roadside inspections. In the 2025 International Roadcheck, brake systems and brake adjustment together made up over 40% of vehicle OOS violations. A daily air brake check takes a few minutes and catches most problems before an officer does.
How the system works, in short
The engine-driven compressor fills air tanks. A governor stops the compressor when pressure reaches cut-out, often around 120–140 psi depending on the truck. When you press the pedal, air pushes the brake chambers, which move the pushrods and slack adjusters to apply the brakes. Spring brakes (parking and emergency) are held off by air and apply when air is lost.
Seeing a light on the dash? Snap it with DERATE and know if you can keep driving
The daily air brake check
Always follow your CDL manual and your truck's specs. A common sequence:
- Air build-up. With the engine at idle, pressure should rise from 85 to 100 psi within about 45 seconds on dual-tank systems, or within your manufacturer's spec.
- Governor cut-out. Note the pressure where the compressor stops. Check it against your truck's spec.
- Applied leakage test. Engine off, brakes released, fully apply the pedal and hold for one minute after the first drop. Loss should be no more than 3 psi on a single vehicle or 4 psi on a combination.
- Low air warning. Fan the brakes to bring pressure down. The warning buzzer or light must come on at or above 60 psi.
- Spring brakes and tractor protection. Keep fanning. The tractor protection valve and parking brake knobs should pop out, usually between 20 and 45 psi.
- Parking brake test. With the parking brake set, gently try to pull forward. The truck should not move.
- Service brake test. At about 5 mph, apply the brakes firmly. The truck should stop straight without pulling.
Common air brake problems
Air leaks
Signs: pressure drops fast, the compressor runs often, you hear hissing. Common spots: gladhand seals, chafed hoses, fittings, brake chambers and valves.
Brakes out of adjustment
Pushrod travel beyond the legal limit is a leading roadside violation. Automatic slack adjusters should keep brakes in adjustment. If one keeps going out, don't just readjust it. Something is wrong: worn parts, poor lubrication or a failed adjuster. Have a mechanic find the cause.
Moisture and oil in the system
Water and oil damage valves and seals. Drain tanks if you have manual drains, and keep the air dryer cartridge serviced.
Worn linings and drums
Check for thin linings, cracks, grease contamination and heat damage.
Spring brake problems
A dragging or stuck spring brake can overheat a wheel end. A broken spring is dangerous. Leave chamber repair to a qualified technician.
The ABS lamp and ABS codes
At key-on, the ABS lamp should light briefly, then go out. That's the bulb check.
- Stays on: ABS has a fault and is turned off. Normal braking still works, but without anti-lock.
- Never lights: the bulb or circuit may be bad. That can also be a violation.
- Comes and goes: usually a loose connector, wiring or sensor gap problem.
Tractor or trailer? Disconnect the trailer and cycle the key. If the dash lamp goes out, the fault is likely on the trailer, often at the 7-way connector. Trailers also have their own external ABS lamp.
Most ABS faults point to a wheel speed sensor, its wiring, or the tone ring. The code usually names the wheel. ABS codes can be read with a J1939/J1587 scan tool or with blink codes on many systems.
An ABS lamp alone usually isn't an out-of-service item, but it's recordable. It can add to OOS if combined with other brake defects.
When to stop and call a mechanic
Stop safely and call for service if the low air warning comes on while driving, if pressure won't build, if the spring brakes start to apply, if the truck pulls hard to one side, or if you smell hot brakes or see smoke from a wheel. Don't drive with leaks that fail the leakage test or brakes out of adjustment.
FAQ
At what pressure should the low air warning come on?
At or above 60 psi.
How much air loss is allowed in the leak test?
With brakes fully applied and engine off: no more than 3 psi in one minute for a single vehicle, or 4 psi for a combination.
Can I drive with the ABS light on?
Your regular brakes still work, but ABS doesn't. Drive carefully, get it fixed soon, and expect it to be written up at an inspection.