8.2 Inspection Set-Up and Recognizing Air Brake Defects
Key Takeaways
- Choose level ground away from traffic, set the parking brakes, and chock the wheels so the chocks will hold the vehicle when the air brakes are released; square chock blocks should be at least 15 by 15 cm.
- Look directly at pushrods and clevis pins: broken pushrods and clevis pins whose cotter pins have sheared can be hard to spot while the parts are still lined up.
- Drum brake linings must be at least 1/4 inch thick (CVSA); linings worn thinner can let the S-cam roll over (cammed over), leaving no braking at that wheel.
- Oil from a leaking wheel seal streaks around the inner sidewall of the inner tire if it leaks while moving, and drains down below the axle and puddles if it leaks while parked.
- Corroded spring brake chambers or clamp rings can release the park spring with deadly force and must be repaired at once by certified technicians; blocking off air lines is never acceptable.
Inspection Set-Up and Recognizing Air Brake Defects
Quick Answer: Before any air brake inspection, park on level ground away from traffic, set the parking brakes, put a manual transmission in low gear, shut off the engine, and chock the wheels so they will hold when the brakes are released. Square chock blocks should be at least 15 × 15 cm. Then inspect the air brake components by looking directly at each part. Watch for broken pushrods, clevis pins working out, linings under 1/4 inch or cammed over, oil-soaked linings, cracked drums or rotors, slack adjusters coming off their splines, chafed, bulging, or kinked hoses, mixed fittings, corroded or loose chambers, loose tanks, and blocked-off lines.
Safe set-up
The Commercial Driver's Guide sets out how to begin:
- Choose terrain that is as level as possible and park safely away from traffic.
- Set the parking (spring) brake. Put a manual transmission in low gear.
- Shut off the engine.
- Chock the wheels, and make sure the chocks will keep the vehicle from moving. This matters most on air-brake vehicles, because the brakes will be released later in the inspection. The minimum size for square blocks is 15 by 15 centimetres.
If you will work under the vehicle with its brakes released, remove the ignition key and remember that a transmission left in gear is not a substitute for chocks (Section 6.3). When disconnecting live air lines, keep your hand between the coupler and your face, and wear appropriate personal protective equipment.
What to look at: the visual air brake inspection
The course uses photographs from the CVSA North American Standard Out-of-Service Criteria Handbook and Pictorial, the book enforcement officers use in Level 1 inspections. The student manual's margin notes give tips for spotting each defect.
Pushrods and clevis pins
- Broken pushrods: neither a service nor a park application can deliver force to the foundation brake. If the pushrod and slack adjuster are still lined up, the damage can be hard to notice, so look directly at the pushrods.
- Clevis pin failures: the cotter pins shear, the clevis pin works its way out, and the brake stops working when the clevis separates completely. A pin that has only started to back out still lets the brake apply normally, which is why you have to look. Failures happen on new vehicles as well as old ones.
Linings
- Lining failure reduces the friction surface. How much force is lost depends on how much lining is missing, and the brake keeps getting worse as material separates.
- Excessively worn linings can lead to a brake that is cammed over. The lining is so thin that nothing stops the rollers going past the largest radius of the S-cam lobes, and the brake becomes ineffective. CVSA standards require at least 1/4 inch of lining.
- Oil saturation comes from leaking wheel seals and shows in two ways:
- Leaking while moving: oil streaks in radial lines around the whole inner sidewall of the inner tire.
- Leaking while parked: oil drains down the inner sidewall below the axle and puddles on the ground.
Drums and rotors
- Cracked drums come from repeated heating and cooling. Cracks start at the friction surface and work outward, and centrifugal force widens them until the drum comes apart. Pieces usually separate while the vehicle is moving and become dangerous projectiles, and the driver generally does not know when or where it happened. Tell short, fine heat-check lines apart from real cracks, and report any crack that opens or reaches an edge.
- Rotor cracks are easy to see unless backing plates cover the rotor or the crack is hidden under the calliper. Check rotors at every inspection and during the shift.
Slack adjusters
- If the retaining snap ring comes off the end of the S-cam shaft, the slack adjuster can work its way off the splines. The brake works normally until the moment it separates, and then it does not apply at all.
- Rust can weaken a slack adjuster until a brake application breaks it off.
- Check each slack adjuster's travel, mechanical condition, and wear by pulling it by hand or with a pry bar (Section 6.3).
Hoses, lines, and fittings
| Defect | Why it matters |
|---|---|
| Chafed air lines (rubbing on nearby parts because they are badly secured) | No loss of braking until the hose ruptures, which causes a circuit failure (Chapter 5) |
| Bulging hoses (internal structural failure or heavy chafing) | A bulging hose will rupture, causing a circuit failure |
| Kinked hoses | They restrict or block airflow. A kinked plastic line is permanently damaged and must be replaced; a kinked rubber line can have internal damage that later bulges |
| Wrong or mixed fittings | Fitting systems are designed for particular hoses, are not compatible with each other, and must never be mixed |
Glad hands and trailer lines should also be checked: good seals, undamaged couplers, dummy couplers used when no trailer is connected, and lines supported so they will not catch as the trailer turns.
Brake chambers
- Corrosion on spring brake chambers or clamp rings weakens them until the park spring can escape with tremendous force. This is an extremely dangerous failure that can cause severe injury or death. Any corrosion or damage in chambers or clamp rings must be repaired immediately by certified heavy equipment technicians.
- Separated brake chambers give no application at that brake.
- Loose brake chambers still apply some force, but less, because the chamber deflects on its failed mounting. Check the mounting fasteners and brackets.
Other foundation brake parts
Failures range from a small loss of force to complete failure: disconnected return springs, separated rollers, a separated inner S-cam bracket, and a separated brake shoe.
Air tanks
A loose air tank does not cause a failure until it separates from the circuit it supplies, and then the circuit fails immediately. During the inspection, push on each end of each air tank to confirm it is securely mounted.
Blocked-off air lines
Some people deal with an air system failure by blocking off the lines to the failed part. Examples include kinking or wiring a line shut, clamping it with pliers, or putting a coin in a swivel fitting to seal off a leaking chamber. This reduces the vehicle's braking capacity and is never acceptable. If a failure affects braking ability, the vehicle must be out of service until it is properly repaired.
Quick checklist
| Area | Look for |
|---|---|
| Set-up | Level ground, parking brakes set, engine off, wheels chocked (15 × 15 cm minimum square blocks) |
| Compressor and belt | Secure mounting; belt condition and tension (Section 2.1) |
| Tanks | Secure mounting (push each end); drain lanyards; safety valve |
| Chambers | No corrosion, cracks, or looseness; clamp rings sound; nothing blocking pushrod travel |
| Pushrods, clevises, slack adjusters | Nothing broken or missing; cotter pins in place; travel and condition checked |
| Linings, drums, rotors | At least 1/4 in of lining; no oil; no cracks |
| Hoses and fittings | No chafing, bulges, or kinks; correct fittings; no blocked lines |
| Couplers | Seals and couplers good; lines supported |
What does the term 'cammed over' describe in a drum brake?
Oil from a leaking wheel seal has streaked in radial lines around the entire inner sidewall of an inner tire. What does that pattern tell you?
A driver finds a coin wedged in a swivel fitting to stop air leaking from a damaged brake chamber. What does the Alberta course say about this?