The most basic symptom of a small leak is a compressor that runs too much. If it runs 30 percent or more, or if you hear it cycling while sitting in traffic or when idling with parking brakes applied, the air supply is leaking.
To confirm, shut down and depressurize the system. Tee a gauge into the wet tank. Idle until air pressure is up. A rapid leak-down and rise of pressure confirms a supply leak.
1. To locate leaks audibly, get into an enclosed, quiet area. Idle until the air compressor cuts off, then shut down.
2. Examine the compressor discharge line for copper cracks and for moisture and oil. Check hoses for rubbing. Inspect the compressor unloader and the dryer purge valves. You can spray soapy water on connections and look for bubbles.
3. Look for parking system leaks. With wheels chocked, have a helper engage and disengage the parking brake repeatedly. Watch the spring brakes on each axle retract. One retracting slowly or partially means a leak. Check for a drum on one wheel that runs hot even when others are cool, which means incomplete release. Use soapy water to check parking brake hoses and connections with the parking brakes released.
4. Have an assistant repeatedly apply and release service brakes. A slow application of one wheel’s brakes, or one axle, may indicate a significant leak in the line or chamber serving that wheel or in the axle plumbing.
5. Trace all lines using soapy water. Note that service hoses feed the inner chambers closer to the slack adjusters, and the connecting plumbing. Check all connections, relay valves and hoses.
6. If you have a leaky relay valve exhaust port, check for a leaky brake chamber. Depressurize the service system and disconnect and plug each of the valve’s ports, then restore pressure one port at a time. If plugging one port stops the leak at the exhaust port, the problem is the brake chamber fed by that port. If the leak continues, it’s the relay valve (rear in photo, behind brass fitting). n