Air Brake Fault Diagnosis for Commercial Drivers

You already know how an air brake system works and how to run the standard checks. On the road, the question is different. A gauge is falling. A trailer will not release. A wheel smells hot. You have to work out what that means, decide whether the truck can move, and explain it clearly to the mechanic on the phone.

This is about the driver's side of the problem: reading symptoms and making a safe, legal decision. Air brake repair belongs to a qualified brake inspector or mechanic. Under 49 CFR 396.25, anyone responsible for brake service or repair must be qualified for that task. Your carrier will set its own procedures too, so follow both.

Warning: Never take apart a spring brake chamber. The spring inside holds enough force to kill. Leave any chamber work, caging and brake adjustment to someone trained and authorized to do it.

What You Will Be Able to Do

• Sort an air brake symptom into stop now, do not move, or report.
• Read pressure gauges to tell a leak from a compressor or governor fault.
• Work out why trailer brakes will not release or why they drag.
• Judge a brake adjustment problem, and explain why adjusting an automatic slack adjuster by hand is the wrong fix.
• Report a defect so that it is fixed and you are protected.

Decide First: Can It Move?

Before you look for the cause, decide how urgent this is.

Under 49 CFR 396.7, a vehicle may not be run in a condition likely to cause an accident or breakdown. Under 396.11 you report defects in writing at the end of the day. Under 396.13 you have to be satisfied that the vehicle is in safe condition before you drive it. If in doubt, it does not move.

Reading Your Gauges

Your dual gauge is your best diagnostic tool. Watch what each needle does, and when.

Worked Example: Reading a Failed Leak Test

You build the air to governor cut-out on a tractor-trailer. With the engine off, the brakes released and the pressure settled, you see a drop of 2 psi in a minute. That is fine, because the limit is 3 psi for a combination. Next, you hold the pedal down firmly. After the first drop, pressure falls 7 psi in one minute. The limit is 4 psi for a combination.

So the system holds with the brakes released, but it leaks when they are applied. That points to the service side, such as a brake chamber diaphragm, a service line, a foot valve or a trailer service circuit. Have a helper hold the pedal while you walk the vehicle and listen for air at each wheel and at the trailer. Do not move the truck until a mechanic has fixed it.

When the Compressor Is the Suspect

Air that does not build could be a failed compressor, a broken drive or a fault in the air dryer. Listen to the dryer. A short burst of air when the governor cuts out is the dryer purging, and that is normal. A constant hiss from the dryer's purge valve is not. Note what you hear when you call it in, because that helps the mechanic.

Trailer Problems

Most on-the-road air brake calls involve the trailer. Think through the two lines.

Crossed lines. With the red and blue lines swapped, supply air goes to the service line and does not charge the trailer tanks, so the spring brakes will not release. An older trailer without spring brakes is worse, because you could drive away with no trailer brakes at all. Check the color coding after each coupling. Then test the trailer brakes with the hand valve or the air supply knob, and pull gently against them in a low gear.

Torn lines on the road. If the red supply line tears off, the trailer loses air, and its spring brakes apply on their own. If the blue service line tears off, you may notice nothing until you press the pedal. Then air escapes at the break, the pressure falls, and the warning comes on. If it falls low enough, usually in the range of 20 to 45 psi, the tractor protection valve closes and the trailer's emergency brakes come on. Slow down, and stop safely as soon as you notice air loss.

Frozen Brakes

Water in the air lines freezes. Wet brakes that were parked in freezing weather can also freeze to the drums.

1. Check that air pressure is at normal, and that the parking knobs are pushed in.
2. If the wheels stay locked, the brakes are likely frozen. Do not try to break them free with engine power.
3. Do not use an open flame to thaw lines or brakes. Fire can damage air lines, seals and tires, and it can start a fire.
4. Follow your carrier's approach, or call for help.

You can often prevent this. If the brakes are wet, use them lightly in a low gear to heat and dry them before you park. CDL manuals say not to set the parking brakes when the brakes are very wet in freezing weather. Hold the truck with wheel chocks on a level surface instead.

Drain your tanks as your carrier requires. Have the air dryer checked if moisture appears at the drains. Do not add anything to the air system unless the maker and your carrier say to.

Heat, Fade and Hot Wheels

As brakes heat up, drums expand, and the pushrod has to move further to reach them. A brake that is only just in adjustment when cold can be out of adjustment when hot. So brakes that work on a flat road can fail on a long hill.

Tires can catch fire from a dragging brake, and a tire fire is very hard to put out. Keep your distance if flames take hold.

Brake Adjustment and Stroke

The pushrod's stroke is how far it moves when the brake applies. Too much stroke means the brake is out of adjustment. Federal rules at 49 CFR 393.47 set stroke limits by chamber type and size. For a common standard clamp-type Type 30 chamber, the limit is 2 inches. For a long-stroke Type 30, it is 2.5 inches. The size is normally marked on the chamber, and some chambers have a marked limit. Always use the correct limit for the chamber.

Roadside inspectors use the Commercial Vehicle Safety Alliance (CVSA) out-of-service criteria. Under these, a vehicle goes out of service when 20 percent or more of its service brakes are defective.

Worked example: A five-axle tractor-trailer has 10 brakes, one at each wheel end. 20 percent of 10 is 2. If an inspector finds two defective brakes, the vehicle cannot move until it is repaired. So one bad brake on your pre-trip check is one away from being placed out of service. The CVSA criteria are updated each year, and they include details of how brakes are counted.

Do Not Adjust an Automatic Slack Adjuster to Hide a Problem

Most trucks now use automatic slack adjusters. They adjust themselves as the linings wear. A brake with an automatic adjuster that is out of adjustment means something has failed: the adjuster, the foundation brake or something else.

Adjusting one by hand is a false fix. The brake goes back out of adjustment soon, and the fault that caused it stays hidden. Appendix A to 49 CFR Part 396 says it is not acceptable to adjust one by hand without first correcting the underlying problem. That problem can be something like a broken weld, loose mounting hardware or a cracked drum. If a roadside check finds it, tell your carrier that it needs a mechanic, not a wrench.

ABS Warning Lamps

Truck tractors built on or after March 1, 1997, and other air-braked vehicles built on or after March 1, 1998, must have ABS. Tractors have a yellow lamp in the dash. Many trailers have a yellow lamp on the outside, usually on the left side at the front or rear corner. Tractors built on or after March 1, 2001 also have a lamp in the cab for the trailer's ABS.

The lamp should come on for a bulb check, then go out. If it stays on, ABS may be off for that vehicle. Your normal brakes still work, but you may lose control more easily in a hard stop on a slick road. Allow more room, report it and get it fixed. Do not treat it as something to ignore until the next service.

Reporting It Right

A good report gets the truck repaired faster, and it protects you.

1. What: "Applied leak of 7 psi a minute, tractor-trailer," not "Air problem."
2. Where: Which axle, which side, tractor or trailer.
3. When: Cold, after a grade, only when braking.
4. What you did: Stopped, chocked, tested.

Write it on your driver vehicle inspection report. Do not take the truck back on the road until the defect has been repaired or a carrier's agent has certified that repair was not needed. Before the next trip, look at the last report and confirm the repair.

If a dispatcher tells you to run with a known brake defect, you have the right to refuse. Coercion to break safety rules is prohibited under 49 CFR 390.6. Put your concern in writing.

Common Mistakes

Check Your Understanding

1. On a level road, the secondary needle starts falling, and the primary holds at 120 psi. What do you do, and why? Slow down and stop safely. A circuit is losing air, and it may not brake well. Do not drive on until it is fixed.
2. The pressure keeps climbing past 135 psi, and the safety valve pops. What does that suggest? The governor is not cutting the compressor out. Stop and report it.
3. At the shipper, your trailer's wheels stay locked with the red knob pushed in, and the gauge shows 115 psi. What do you check? The glad hands, line connections and seals, whether the lines are crossed, and the trailer's shut-offs.
4. An inspector finds 3 of your 10 brakes defective. What happens? The vehicle goes out of service, because 3 of 10 is 30 percent, over the 20 percent limit.
5. At a fuel stop, you find a pushrod with too much stroke on a brake with an automatic slack adjuster. A coworker offers to wind it in. What do you say? No. The adjuster should not need it. Something has failed, so report it and have a mechanic find the cause.

Key Takeaways

• Decide if the truck can move, then look for the cause.
• Which needle moves, and when, tells you where the fault is.
• A red line loss applies the trailer brakes right away. A blue line loss shows when you brake.
• Heat makes a marginal brake worse, so a pass when cold does not prove it will hold on a grade.
• Out-of-adjustment automatic slack adjusters mean a fault, not a need for a wrench.
• Report the defect in detail, and leave chambers and repairs to qualified people.