Air leakage is defined as measuring the size of a bubble formed over a specified period of time. A 25.4 mm (1 in) bubble formed over 5 seconds is equivalent to 100 standard cubic centimeters per minute (sccm) and a 25.4 mm (1 in) bubble formed over 3 seconds is equivalent to 175 sccm.

Use a soap solution to perform all air leakage tests. This allows a bubble to form at the site of the air leak, giving a visual aid for diagnosing the air loss rate.

1. Inspect all tubing for the following conditions:

•

Kinks

•

Dents

2. Inspect all hoses for the following conditions:

•

Wear

•

Drying

•	Overheating

3. Inspect the suspension of all of the tubing. The tubing must be supported to eliminate vibration.
4. Inspect the suspension of all of the hoses. Position the hoses so that they do not abrade or are subjected to too much heat.
5. Apply the parking brake.
6. Start the engine and charge the air system to the governor cut-out point.
7. Stop the engine.
8. Apply and release the service brakes.
9. Check the air leakage.

•	Check that the dual air pressure gauge on vehicles without trailer controls does not drop more than 28 kPa (4 psi) in 2 minutes.

•	Check that the dual air pressure gauge on vehicles with trailer controls does not drop more than 41 kPa (6 psi) in 2 minutes.

•	If the dual air pressure gauge reading drops more than specified, check for leaks at the air line connections, the application valve, the trailer control valve, the relay valve, the park brake control valve, the governor, and other parts on the supply side of the application valve.

10. Start the engine and charge the system to the governor cut-out point.
11. Stop the engine.
12. Firmly apply and hold the brakes.
13. Check the air leakage.

•	If the air pressure drops 10 percent or more, check the air brake adjustment at each wheel or too much lining and brake drum wear.

•	Check that the dual air pressure gauge on vehicles without trailer controls does not drop more than 41 kPa (6 psi) in 2 minutes.

•	Check that the dual air pressure gauge on vehicles with trailer controls does not drop more than 55 kPa (8 psi) in 2 minutes.

•	If the dual air gauge reading drops more than specified, test for leaks at the air line connections. Double check the valves, the quick release valve, the relay valve and the other parts on the delivery side of the application valve.

•	Start the engine and charge the air system to the governor cut-out point.

•	Stop the engine.





•	On vehicles with trailer provisions, move the trailer air brake control valve handle up to the full ON position.

•	Air leakage should be less than 41 kPa (6 psi). Check the air lines, air line fittings, trailer air brake control valve, towing vehicle protection valve, and any valves on the trailer.

Check the reservoir drain valve, periodically, for leakage. Replace the reservoir drain valve if any leakage exists.

1. Set the parking brake.
2. Start the engine.
3. Charge the air system to the governor cut-out point.
4. Apply a soap solution to cover the reservoir drain valve.
5. If any bubbles appear, indicating air leaks, replace the reservoir drain valve.