Periodically check the reservoir drain valve for leakage. No leakage is allowed. Replace the reservoir drain valve if needed.

1. Apply 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 bubbles appear indicating air leaks, replace the reservoir drain valve. Refer to the appropriate procedure:

•	Primary Air Brake Supply Reservoir Drain Valve Replacement

•	Front Axle Service Reservoir Drain Valve Replacement

•	Rear Axle Service Reservoir Drain Valve Replacement