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You should use the basic seven-step inspection procedure described in "Introduction to Safe Driving" Lesson to inspect your vehicle. There are more things to inspect on a vehicle with air brakes than one without them. These things are discussed below, in the order they fit into the seven-step method.
Check Air Compressor Drive Belt (if compressor is belt- driven). If the air compressor is belt-driven, check the condition and tightness of the belt. It should be in good condition.
Check Slack Adjusters on S-cam Brakes: Park on level ground and chock the wheels to prevent the vehicle from moving. Turn off the parking brakes so you can move the slack adjusters. Use gloves and pull hard on each slack adjuster that you can reach. If a slack adjuster moves more than about one inch where the push rod attaches to it, it probably needs adjustment. Adjust it or have it adjusted. Vehicles with too much brake slack can be very hard to stop. Out-of-adjustment brakes are the most common problem found in roadside inspections. Be safe - check the slack adjusters.
All vehicles built since 1994 have automatic slack adjustors. Even though automatic slack adjustors adjust themselves during full brake applications, they must be checked.
Automatic adjusters should not have to be manually adjusted except when performing maintenance on the brakes and during installation of the slack adjusters. In a vehicle equipped with automatic adjusters, when the pushrod stroke exceeds the legal brake adjustment limit, it is an indication that a mechanical problem exists in the adjuster itself, a problem with the related foundation brake components, or that the adjuster was improperly installed.
The manual adjustment of an automatic adjuster to bring a brake pushrod stroke within legal limits is generally masking a mechanical problem and is not fixing it. Further, routine adjustment of most automatic adjusters will likely result in premature wear of the adjuster itself. It is recommended that when brakes equipped with automatic adjusters are found to be out of adjustment, the driver take the vehicle to a repair facility as soon as possible to have the problem corrected.
The manual adjustment of automatic slack adjusters is dangerous because it may give the driver a false sense of security regarding the effectiveness of the braking system.
The manual adjustment of an automatic adjuster should only be used as a temporary measure to correct the adjustment in an emergency situation as it is likely the brake will soon be back out of adjustment since this procedure usually does not fix the underlying adjustment problem.
Note: Automatic slack adjusters are made by different manufacturers and do not all operate the same; therefore, the specific manufacturer’s Service Manual should be consulted prior to troubleshooting a brake adjustment problem.
Check Brake Drums (or Discs), Linings, and Hoses:
Do the following checks instead of the hydraulic brake check shown in Step 7: Check Brake System.
Check Rate of Air Pressure Buildup: When the engine is at normal operating speed, the pressure should build from 85 to 100 psi within 45 seconds in dual air systems (if the vehicle has larger than minimum air tanks, the buildup time can be longer and still be safe. Check the manufacturer’s specifications.) In single air systems (pre-1975), typical requirements are pressure build up from 50 to 90 psi within 3 minutes with the engine at an idle speed of 600-900 rpm.
If air pressure does not build up fast enough, your pressure may drop too low during driving, requiring an emergency stop. Don't drive until you get the problem fixed.
Check Air Compressor Governor Cut-in and Cut-out Pressures: Pumping by the air compressor should start at about 100 psi and stop at about 125 psi (check manufacturer’s specifications). Run the engine at a fast idle. The air governor should cutout the air compressor at about the manufacturer’s specified pressure. The air pressure shown by your gauge(s) will stop rising. With the engine idling, step on and off the brake to reduce the air tank pressure. The compressor should cut-in at about the manufacturer's specified cut-in pressure. The pressure should begin to rise.
If the air governor does not work as described above, it may need to be fixed. A governor that does not work properly may not keep enough air pressure for safe driving.
Test Air Leakage Rate: With a fully-charged air system (typically 125 psi). Test the leakage rate without the foot brake applied:
Next, test the leakage rate with the foot brake applied. With the air pressure built up to governor cutoff (120 – 140 psi):
Test Low Pressure Warning Signal:
Test Parking Brake. Test the parking brake using the following method:
On Combination Vehicles, also test the trailer parking brakes as follows:
If the brakes do not stop the vehicle from moving forward, they are faulty and must be fixed before you travel.
Test Service Brakes:
Note any vehicle “pulling” to one side, unusual feel, or delayed stopping action. This test may show you problems, which you otherwise wouldn't know about until you needed the brakes on the road.