For 1990-2009 cars

1. Block the wheels.
2. Apply the parking brake.
3. Drain all of the air reservoirs. Refer to Air Brake Reservoir Draining .
4. Remove the air line from the differential lock control valve delivery port.
5. Install an air pressure gage into the delivery port of the differential lock control valve.
6. Shift the differential lock control valve lever to OFF.
7. Start the engine. Charge the system to the air compressor governor cutout point.
8. Stop the engine.
9. Shift the differential lock control valve lever to ON.
10. Verify that the air pressure reading on the air gage is comparable with the vehicle's air pressure gage. If the readings on the gages are not comparable, repair or replace the inter-axle differential lock control valve.
11. Drain all of the air reservoirs.
12. Remove the air pressure gage from the delivery port of the differential lock control valve.
13. Install the air line into the differential lock control valve delivery port.
14. Start the engine. Charge the air system to the air compressor governor cutout point.
15. Stop the engine.
16. Inspect for air leaks using a soap solution. Air leakage is not allowed. If any air leaks are present, repair or replace the parts as needed.