Air Brakes on Trucks/TrailersDid you Inspect

and Adjust based upon

the Standards

Did you COVER it in a Safety Meeting and your Policy Manuals

ALWAYS CHOKE IT UP for SAFETY

MEASURE don’t GUESS

Less than2” of Travel

Air brakes are adjusted by manual slack adjusters located on the air canister on the axle housing.

Step 1: Release the Air in the System• Before beginning it is important to chock the tires to prevent the vehicle from moving

during the adjustment. While performing the adjustment try to keep each push rod at the same distance in order to keep each wheel's stopping ability close to the same. Make sure that the air brakes system is at full pressure, lower the emergency brake, and push in the air valve releasing the brakes.

Step 2: Check the Push rod• Using a piece of chalk, mark the location on the push rod where it leaves the air

chamber. Then using a tool for leverage, pull the push rod out of the brake chamber, and measure from the chalk line to the brake chamber. This will show you the stroke range or how much air must be compressed in order to stop the vehicle. Anything over ¾ of an inch means your brakes need to be adjusted.

Step 3a: Adjusting the Pushrod for Manual Adjusters• Locate the manual slack adjuster near the push rod that you would have checked

earlier. Watching the s-cam to see the direction it is turning will help to make sure you are adjusting it correctly and not loosening it. You can tell you are turning it the wrong way if while turning the push rod is pulling out of the air chamber. Once the proper direction has been discovered, continue tightening until you feel it resist. This means you have met the brake linings to the drum. Reverse the adjuster bolt ¼ to ½ turns to maintain a clear distance from the drum to the linings. If for some reason you can turn the manual adjuster bolt in both directions and cannot tell the difference between loosening and tightening, it is time to replace the slack adjuster.

Step 3b: Adjusting the Pushrod for Automatic Adjusters• If you are equipped with automatic slack adjusters, they need no manual adjustment.

However, it is still important that you check to make sure proper adjustment is maintained in order to catch brake issues before they happen. If you do make an adjustment of them keep in mind that it will temporarily alter the stroke but eventually the slack will revert to its intended stroke. With automatic adjusters you need to have an air pressure of above 90 psi, have the parking break off and the parking brake control valve and supply valve open. Press the brakes completely and release repeating at least 6 times to test the brakes. This process of air brake test will allow the adjusters to fix themselves if needed. If for some reason you have completed all of these steps and the stroke of the push rod is still excessive, manually adjusting the brakes will only assist in getting you to a repair station.

In Canada

Turn it PROPERLY

Canada IMPORTANT Standards

CONTAMINATION AIR-BRAKE CONTAMINATION • Air-brake systems are subject to excessive oil contamination. As reliable as they are,

vehicle compressors can and do fail. One form of announcing this problem to the diagnostic technician is the increasingly larger amounts of oil that are passed on to the wet tank. With this in mind Law Enforcement strongly recommends that timing and balance is checked after cleaning a brake system.

OIL CONTAMINATION• Oil contamination in an air-brake system is one of the problems that can arise that

must be dealt with in a clear and concise manner. The rubber components in the valuing system are made of a specific type of rubber that, when exposed to oil contamination, can cease to function properly. If this happens, the timing of the air brakes on any air-braked vehicle can be compromised and lead to poor or insufficient braking.

• Each valve within the air brake system is designed to operate on a specific pressure. The relay valves, at each position, have built into them through a rubber valve and spring a "crack pressure" to determine when the valve will begin to function. This crack pressure allows for the application of one axle before another axle. This in turn allows the trailer to brake slightly before the tractor drive wheels. It also allows the tractor drive wheels to brake before the tractor steers through crack pressure and the treadle pressure to ensure proper braking of the entire unit.

• If oil contamination swells up those crack-pressure rubber valves, the timing can become improper and allow for brakes to apply or not depending on the amount of damage to the crack-pressure valve and spring. Simply cleaning the valve does nothing to restore the proper crack pressure and therefore does not resolve the problem. Replacement of the valves, in many cases, is the only answer. Cleaning of the rest of the system can be accomplished if it does not re-contaminate the valve and cause the problem to re-occur.

• TIMING & BALANCE MUST BE MAINTAINEDUsing a suitable method of checking timing and balance and careful inspection of the operation of the air brake system after ANY CLEANING are strongly recommended. This will ensure that the air-brake system is within the manufacturer's specifications. Likewise, the air compressor and the air-intake system must be checked to ensure that there are no restrictions that could cause the oil problem in the first place.

Safety Toolbox Discussion