Air Brake Problems: The Symptoms You Should Never Drive Through

Air brakes are honest. They tell you they are in trouble before they fail, and they usually tell you more than once. The drivers who get hurt are almost never the ones who had no warning. They are the ones who had a warning and decided to make the next exit.

Here is what each symptom means mechanically, and where the line sits between annoying and lethal.

The low air warning is a stopping order

That light and buzzer are required to come on at 60 psi or above. They are not a courtesy. They are the last comfortable warning you get.

Keep going and pressure keeps dropping, and somewhere between about 45 and 20 psi the spring brakes apply themselves. Not gently, and not when you choose. If that happens at highway speed, the loaded trailer behind you is now trying to get past you and you have no say in it.

When the buzzer goes off: signal, get off the road, use the brakes you still have while you still have them, and stop. You are not going to beat the leak to the exit.

The compressor that cannot keep up

• Build time. Roughly 85 to 100 psi should take 45 seconds or less on a dual system at a fast idle. Much slower and the compressor is tired, the governor is off, or you are leaking as fast as you build.
• Cut-out and cut-in. The governor should cut out around 120 to 135 psi with a sharp dryer purge, and cut back in about 20 to 25 psi lower. A compressor that never cuts out, or cycles constantly, is a governor or unloader problem and it will cook the compressor.

A compressor that runs continuously also pumps oil into the tanks. Oil swells the rubber in every valve downstream and kills the dryer desiccant, which then stops removing water.

Brakes that drag and wheel ends that cook

This is the one that starts truck fires. A brake that will not fully release keeps rubbing, rubbing makes heat, and heat cooks the lining, the drum, the bearing and eventually the tire.

If you climb out after a run and one hub is noticeably hotter than the rest, or one wheel is smoking, or you smell hot brake, that brake has been dragging for miles. Causes: a seized s-cam or slack adjuster, a spring brake not fully releasing, a stuck relay valve, a broken return spring, or a parking brake that never let go.

Do not touch a hot hub and do not spray water on it to cool it down. A hot wheel end can flash a tire into flame, and cold water on a red-hot drum can crack it. Park it, stay away from it, let it cool on its own, and get someone out to it. If a tire is already burning, get well clear of the truck. Truck tires do not burn quietly.

Brakes that pull, grab, or hang up

If the truck pulls to one side under braking, the brakes are not applying evenly. Usually one chamber is weak or one brake is out of adjustment far enough that it barely contributes while its mate does all the work. That is a torque imbalance, and it is a jackknife waiting for a wet road.

If a brake grabs, suspect lining contaminated by a leaking axle or wheel seal. Once lining is oil-soaked it is finished, and it cannot be cleaned back into service.

If the brakes hang on after you lift off the pedal, air is not getting back out fast enough. That is usually a failing relay valve, a failing quick-release valve, a kinked line, or ice. Drain your tanks, wet tank first. If you pull the petcock and get a stream of water and oil instead of a puff of air, your dryer is not doing its job.

Out of adjustment: the silent one

This symptom makes no noise and lights no lamp. The truck stops just fine at 30 mph in a parking lot. It only shows up when you need every brake you have, and then it is not there.

Chock the wheels, build air, mark the pushrod at rest, make a full application, and measure the travel. On the common long-stroke type 30 chamber the readjustment limit is 2 inches. Past that, the brake is barely working, and it counts against you at a scale house too, where 20 percent defective brakes puts the whole vehicle out of service.

Fade on a grade

Brakes turn motion into heat. That is the whole job. Overheat them and the lining glazes, the drums expand away from the shoes, and the pedal stops doing anything. More pedal makes it worse.

The fix is technique, not parts. Pick your gear before the hill, not during it. Let the engine brake carry the load. Use the service brakes in firm, short snubs down to a target speed, then release and let them cool, instead of riding light pressure all the way down.

Never fan the brakes on a long grade. Pumping the pedal on an air system bleeds pressure out of the tanks faster than the compressor can replace it, and you can chase yourself all the way down to the pressure where the spring brakes apply on their own, halfway down a hill, with nothing left.

The list you never drive through

• Low air warning light or buzzer.
• Air pressure dropping while you drive, or a compressor that will not build back to cut-out.
• A leak you can hear standing next to the truck.
• Any hot hub, smoking wheel, or hot brake smell.
• Pulling hard to one side under braking.
• A soft pedal, a long pedal, or brakes that hang up on release.
• Any brake you have measured past its stroke limit.

And two things you never do to keep moving: never cage a spring brake so you can drive a truck with a failed air system, and never plug or bypass a leaking line to hold pressure. The springs are the only brake left when the air is gone. Give those up and you are not driving a truck anymore.

If the air system is telling you something and you cannot fix it where you sit, that is the call to make. Duckett Roadside Repair works air systems, brakes and air leaks out of a fully-loaded service truck across Milton, Pace, Pensacola, Navarre, Crestview and the I-10 corridor. Dispatch answers at (850) 495-0366, 24 hours a day, 7 days a week. Get the truck off the road, get the triangles out, and call.