Perform the air buildup test in the following sequence:

1. Before starting the engine, open the air drain cocks and release the air pressure from the system.

2. Close the air reservoir air drain cocks (fig. 6- 11).

3. Start the engine and watch the air pressure gauge to see how long it takes to build up to safe operating pressure. If it takes longer than 10 minutes to bring the air pressure from 0 to 60 psi, check the system for leaks, and check the air compressor and relief valves.

Conduct the air leakage test with the air brake system at normal operating pressure and the engine turned off. Hold the air brakes in the maximum applied position and watch the air pressure gauge on the dashboard of the vehicle. The air pressure should not drop more than 3 pounds in 1 minute after the brakes are applied and 2 pounds in 1 minute with the brakes released. If the indicated air pressure drops more rapidly than the times specified here, there is an air leak in the system. Trace the air lines to determine the exact source of the leak. Since air leaks normally make a distinct hissing sound, when you find the source of the noise and you have found the leak. Smaller leaks are not as audible and are harder to detect; however, you can detect these leaks by brushing the hose or tubing connections of the air brake system with a solution of soapy water. Air bubbles indicate a leak.

Air brakes on trailers get an external brake inspection as part of the inspetion required on a truck-trailer combination. They are also tested for holding as if the trailer were suddenly disconnected from the tractor. To conduct this test, first make sure the air lines between the tractor and trailer are coupled properly. Then, after you start the engine so both tractor and trailer air reservoirs are charged, quickly and simultaneously disconnect both air line couplings. The trailer or semitrailer brakes should be automatically applied. Trailer brakes are designed to stop the trailer when it is accidentally disconnected from the towing vehicle. All states require automatic application of trailer brakes in an emergency. Some states go even further for trailers having a chassis and body weight of 1,000 pounds or over; such trailers must be equipped with adequate brakes that will also hold the vehicle for at least 15 minutes after application.

If these inspections and tests do not disclose the fault, consult the troubleshooting chart of table 6-3.

Figure 6-11. - Air reservoir with air drain cock.