Air leakage is defined in this manual as measuring the size of a bubble formed over a specified period of time.

1. A 25 mm (1 in) formed over 5 seconds is equivalent to 100 standard cubic centimeters per minute (sccm).
2. A 25 mm (1 in) bubble formed over 3 seconds is equivalent to 175 sccm.

1. Block the wheels.
2. Start the engine.
3. Charge the system air pressure to the governor cut-out point.
4. Pull out the system park knob (3) and release the brake pedal.
5. Verify the spring brake chamber assemblies are applied.
6. Push in the system park knob (3).
7. Verify the spring brake chamber assemblies are fully released.
8. Turn off the engine.
9. Drain the front service reservoir. Refer to Draining Reservoirs .
10. Observe the dual air pressure gauge while the rear reservoir is draining.
11. Verify that the system park knob (3) pops out when the air pressure is 138-310 kPa (20-45 psi). If the knob (3) does not pop out.
12. Replace the valve (7). Refer to Instrument Panel Mounted Air Brake Park Control Valve Replacement or overhaul the valve. Refer to Air Brake System Description and Operation .
13. Remove the wheel blocks.