CDL · Air Brakes · 6 min read
Checking Brake Hardware and Adjustment
- Identify the visual and physical inspection requirements for air brake hardware, foundation brakes, and slack adjusters across the standard inspection steps.
- Explain the dangers of manually adjusting automatic slack adjusters and describe the correct procedures for diagnosing out-of-adjustment conditions.
- Perform the complete Step 7 air brake test sequence, including air compressor governor settings, static and applied leakage limits, low pressure warnings, spring brake activation, and operational checks.
Air Brake Components in the Seven-Step Inspection
Inspecting an air brake system fits directly into the basic seven-step inspection procedure used for all commercial vehicles. Because pressurized air powers these systems through specialized valves, lines, and actuators, vehicles equipped with air brakes have more inspection items than vehicles without them. You must know when and where to inspect each air brake component during your regular walk-around.
During Step 2 of the seven-step inspection, check the air compressor drive belt if the compressor is belt-driven. If the air compressor is belt-driven, you must check the condition and tightness of the drive belt. If the compressor is belt-driven, check the drive belt’s condition and tightness; it should be in good condition.
Later in the walk-around, during Step 5, you inspect the foundation brake parts at each wheel. Brake drums or discs must not have cracks longer than one-half the width of the friction area. Brake linings must not be loose, must not be soaked with oil or grease, and must not be dangerously thin. Look closely at the air lines as well; air hoses connected to brake chambers must not be cut or worn due to rubbing. All mechanical brake parts must be in place and not broken or missing.
Common mistakes
- Attempting to check slack adjuster free play while the parking brakes are applied. Park on level ground, chock the wheels, and release the parking brakes so you can move the slack adjusters during the check.
- Routinely manually adjusting an automatic slack adjuster instead of taking the vehicle to a repair facility. Manual adjustment generally masks an underlying problem, and routine adjustment of most automatic adjusters will likely cause premature wear of the adjuster itself.
- Confusing the static leakage test conditions with the applied leakage test conditions. Static leakage is tested with the foot pedal released, while applied leakage requires fully applying and holding the foot brake for one full minute.
- Forgetting that low air pressure warning devices in dual air systems must activate before the tank with the lowest pressure drops below 55 psi, not the average of both tanks.
Key points
- Air brake checks integrate into the seven-step inspection: compressor drive belts are checked in Step 2, foundation hardware and slack adjusters in Step 5, and in-cab functional checks replace the hydraulic check in Step 7.
- To check S-cam slack adjusters, park on level ground, chock wheels, release the parking brake, wear gloves, and pull hard; movement of more than about one inch indicates the adjuster probably needs adjustment.
- All commercial vehicles built since 1994 have automatic slack adjusters, which must still be inspected; manual adjustment should only be a temporary emergency measure because it masks defects and gives a false sense of security.
- Brake drums or discs must not have cracks longer than half the friction area width, linings must not be loose, oily, greasy, or dangerously thin, and mechanical parts must not be missing or broken.
- The air compressor governor cut-out is about 125 psi (tested at fast idle) and cut-in is about 100 psi (tested by stepping on and off the pedal at idle).
- One-minute leakage limits are less than 2 psi static and maximum 3 psi applied for single vehicles, and less than 3 psi static and maximum 4 psi applied for combination vehicles.
- Low-air warnings must activate before pressure drops below 55 psi; the specified valves should pop out at the manufacturer's specified pressure of 20 to 45 psi; and, at operating rpms, dual-system pressure should build from 85 to 100 psi within 45 seconds unless larger-than-minimum tanks permit a longer time under the manufacturer's specifications.
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Sample lesson: Making and Storing Compressed Air25 practice questions on this exam
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