A semi can weigh 80,000 pounds with a load, and every pound of it stops because of air. When an air brake system starts to fail, it rarely does it all at once. It gives you small signals first. Drivers who catch those signals in the yard pay for a small repair. Drivers who ignore them often pay for a tow, a failed inspection, or worse.
This guide covers the problems we see most at our shop on Irving Blvd in Dallas, how to tell what is going on, and when the right answer is to stop driving.
How an air brake system works, in plain words
The air compressor on the engine fills the air tanks. A governor tells the compressor when to stop, usually around 125 psi, and to start again when pressure drops to roughly 100 psi. An air dryer pulls moisture out of that air. When you press the brake pedal, air flows to the brake chambers at each wheel. The chamber pushes a rod, the rod turns a slack adjuster, and the brake shoes press against the drum.
The spring brakes work the opposite way. They are held off by air pressure. If the pressure drops too low, the springs apply the brakes. This is the safety design: lose air, and the truck stops. It is also why a leak is never a small thing.
Warning signs you can notice before a failure
Air pressure that builds slowly or drops fast
Watch your gauges when you start the truck. If the compressor takes a long time to build pressure, it may be worn or there may be a leak somewhere. If pressure falls quickly with the engine off, you have a leak.
A common test used for the CDL air brake exam is the leakage check. With the engine off and the brakes released, a single vehicle should not lose more than about 2 psi in one minute, and a combination about 3 psi. With the brakes fully applied, the limits are about 3 psi for a single vehicle and 4 psi for a combination. If your truck loses more, do not just top it off and go.
A hissing sound
A hiss near the wheels, under the cab, or at the glad hands after you set the brakes usually means air is escaping. Common sources are worn rubber seals on the glad hands, a loose fitting, a cracked or chafed air line, or a damaged brake chamber diaphragm.
One trick that helps: spray soapy water on a suspected fitting with the system pressurized. Bubbles show you where the air is going.
The low air warning
The low air buzzer and light must come on before pressure falls below 60 psi. If you see it in normal driving, something is wrong. Pull over safely and find out why. If pressure keeps dropping, the spring brakes can begin to apply, and that is usually between 20 and 45 psi.
A soft pedal or a longer stopping distance
If the pedal feels spongy, or the truck takes longer to stop than it used to, the brakes may be out of adjustment, the shoes may be worn thin, or air may be leaking. Out-of-adjustment brakes are one of the most common reasons trucks are put out of service at roadside inspections.
The truck pulls to one side
If the truck drifts left or right when you brake, one side is doing more work than the other. That can be a stuck brake, a bad chamber, a contaminated shoe, or uneven adjustment. Do not wait on this one. Uneven braking is hard on tires and dangerous on wet pavement.
Brakes that drag or smell hot
A hot smell, smoke, or a wheel that is much hotter than the others after a drive can mean a brake is not releasing. A seized slack adjuster or a weak return spring can cause it. Dragging brakes burn through shoes and drums and can start a fire.
Water or oil in the air system
If you see water when you drain the tanks, the air dryer may not be working. Moisture in the lines freezes in cold weather and corrodes valves in warm weather. If you see oil in the tanks, the compressor may be passing oil, which damages seals and valves down the line.
What a shop checks during an air brake inspection
When a truck comes in with brake complaints, we do not start by replacing parts. We start by finding the cause. A basic air brake check includes:
- Building pressure and checking the compressor build-up time and governor cut-in and cut-out
- Static and applied leakage tests to see how fast the system loses air
- Walking every air line, fitting and hose for chafing, cracks and wet spots
- Measuring brake chamber pushrod travel and checking the slack adjusters
- Pulling a wheel when needed to measure lining thickness and check drums for cracks and heat damage
- Checking the air dryer and draining the tanks to look for water and oil
- Testing the low air warning and the spring brakes
Then we tell you what we found and what it costs before we start the repair. That is how we work on every job, brakes or not.
When to stop driving
Some problems can wait until you reach a shop. These cannot:
- The low air warning will not go away, or pressure keeps falling while you drive
- You cannot build air back to the normal range
- The spring brakes apply on their own or will not release
- The pedal goes toward the floor, or the truck does not stop the way it should
- A brake is smoking or glowing hot
- You can hear or see a major air leak that does not stop
If any of these happen, get the truck off the road in a safe spot and call a shop. Driving on a failing air system is a risk to you and to everyone around you. If you break down, we try to repair the truck on the spot. If it cannot be fixed roadside, we tow it to our own shop for the repair. We do not offer towing as a separate service.
How to prevent most air brake problems
- Drain the air tanks as part of your regular routine so moisture does not build up
- Do a walk-around before each trip and listen for leaks with the brakes applied
- Replace worn glad hand seals before they fail
- Keep brakes adjusted and have them inspected on a regular schedule
- Fix small leaks early, because they put extra load on the compressor
A few minutes in the yard is cheaper than a day on the shoulder.
Need your brakes checked in Dallas?
If your semi or trailer has any of the signs above, bring it to Bawa’s Truck & Trailer Repair at 4700 Irving Blvd, Ste 200, Dallas, TX 75247. Our brake and suspension repair page lists what we do, and our truck repair team can look at the rest of the truck while it is in. If you are stuck on the road, read about truck roadside repair first, then call (214) 400-4228.
We are open Monday to Saturday, 8 AM to 6 PM. We tell you the price before we start any work.