Testing Air Pressure and Brake Response

Pre-Trip Foundation Inspection and Slack Adjusters

Air brake systems are inspected using the basic seven-step inspection procedure. Because of the specialized mechanical and pneumatic components involved, a vehicle with air brakes has more inspection items than a vehicle without air brakes. During Step 2 engine compartment checks, you must inspect the air compressor drive belt if the compressor is belt-driven. A belt-driven air compressor belt must be checked for condition and tightness, and the air compressor belt should be in good condition before you operate the vehicle.

During the Step 5 walk-around inspection, check the brake drums or discs, linings, mechanical parts, and air hoses; check the slack adjusters on S-cam brakes. Check the brake drums or discs carefully; they must not have cracks longer than one-half the width of the friction area. Inspect the brake linings to ensure they are not loose, dangerously thin, or soaked with oil or grease. All mechanical parts of the brake system must be in place and not broken or missing, and the air hoses connected to the brake chambers must not be cut or worn due to rubbing against other vehicle components.

Step 5 also includes inspecting the slack adjusters on S-cam brakes. To check slack adjusters safely, park on level ground and chock the wheels to prevent vehicle movement. Parking brakes must be released so you can move the slack adjusters by hand. Wear gloves and pull hard on each slack adjuster that can be reached. A slack adjuster probably needs adjustment if it moves more than about one inch where the push rod attaches to it. Inspecting this carefully is essential because vehicles with too much brake slack can be very hard to stop, and out-of-adjustment brakes are the most common problem found in roadside inspections.

The manual states that vehicles built since 1994 have automatic slack adjusters; in this air-brake inspection, check them even though they adjust themselves during full brake applications. However, automatic slack adjusters must still be checked even though they adjust themselves. Automatic slack adjusters should not have to be manually adjusted except when performing brake maintenance or installing the slack adjusters. An automatic adjuster pushrod stroke exceeding the legal limit indicates a mechanical problem in the adjuster, foundation brake components, or improper installation. Manually adjusting an automatic adjuster generally masks a mechanical problem rather than fixing it, and routine manual adjustment of most automatic adjusters will likely cause premature wear of the adjuster itself. Manual adjustment of automatic slack adjusters is dangerous because it can give the driver a false sense of security, so it should only be used as a temporary emergency measure. Drivers should take a vehicle with out-of-adjustment automatic adjusters to a repair facility as soon as possible. Because adjuster designs vary, specific manufacturer service manuals should be consulted before troubleshooting automatic slack adjusters.