The final air brake check is a series of tests, including the governor's cut-in and cut-out pressures, the air leakage rate, the low-pressure warning, automatic spring brake application, and the parking and service brakes. A vehicle that fails any of them should not be driven until it is repaired.
The warning means air pressure has fallen below a safe level, and if it keeps dropping, the spring brakes can come on by themselves, usually somewhere between 20 and 45 psi. Stop in a safe spot while you still have control rather than waiting for the brakes to lock on in traffic.
In this brakes-applied test, a combination vehicle may lose less than 4 psi each minute; the brakes-released limit is lower, at 3 psi. A quicker pressure drop points to a leak that needs fixing before the trip.
Vehicles with air brakes must have a low-air warning, usually a red light that may come with a buzzer, which turns on before tank pressure drops below 60 psi; some manuals give the figure as 55 psi. If it comes on, stop and park safely as soon as you can.
Each of the two circuits in a dual air system must be charged to no less than 100 psi before the vehicle moves. If you start out with less, there may not be enough air for a quick, hard stop.
On vehicles with dual parking control valves, pushing in a spring-loaded control sends air from the separate tank to release the spring brakes, so you can move the vehicle a short way in an emergency. That tank holds only enough air to do this a few times, so plan the move carefully.
Compressed air carries water and compressor oil that settle at the bottom of the tanks, and if they are not removed they can freeze in cold weather or otherwise make the brakes fail. When the tanks have manual drain valves, the driver must empty them after every day of driving.
With the brakes held on and the engine off, a combination vehicle should lose less than 4 psi per minute after the first pressure drop, while a single vehicle should lose less than 3 psi. A faster loss points to a leak that must be fixed before you drive.
An ordinary stop with air brakes is made by pressing the brake pedal and controlling the pressure so the vehicle slows to a smooth stop; with a manual transmission, wait until the engine is close to idle before pushing in the clutch. The parking brake control is for parking, not for routine stops.
On newer air-brake vehicles, you set the spring parking brakes by pulling out a yellow, diamond-shaped knob and release them by pushing it back in. Some older vehicles use a lever for the same job.
When you press the pedal, compressed air has to travel through the lines before the brakes engage, which takes a little longer than with hydraulic fluid. This short delay, called brake lag, adds distance to every stop in an air-brake vehicle.
Air brakes need about half a second of brake lag before they take hold, which adds roughly 32 feet at 55 mph to the perception, reaction and braking distances. Altogether, the stop takes more than 450 feet even on dry pavement with good brakes.
In a vehicle with air brakes, a stop happens in four stages: recognizing the hazard, moving your foot to the pedal, waiting for air to reach the brakes, and the braking itself. The ground covered during all four stages adds up to the total stopping distance.
When the brakes are released and the engine is off, a combination vehicle must lose under 3 psi per minute, while a single vehicle must stay under 2. A bigger drop means the air system is leaking and needs repair before the trip.
Pressing the brake pedal while the spring brakes are set adds air-pressure force on top of the spring force, and the two together can damage the brakes. Not every system is built to prevent this, so make a habit of leaving the pedal alone.
Every question in this set has its own page with the correct answer and an explanation.
