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 two speed rear axle shift control valve delivery port.
5. Install an air pressure gage into the delivery port of the two-speed rear axle shift control valve.
6. Shift the two-speed rear axle shift control valve lever to LOW.
7. Start the engine. Charge the air system to the air compressor governor cutout point.
8. Stop the engine.
9. Shift the two speed rear axle shift control valve lever to HIGH.
10. Verify that the air pressure reading on the air gage is comparable with the vehicle's air pressure gage. If the air pressure reading is not comparable, repair or replace the two speed rear axle shift control valve.
11. Drain all of the air reservoirs.
12. Remove the air pressure gage from the delivery port of the two speed rear axle shift control valve.
13. Install the air line into the two speed rear axle shift 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. If an air leak is present, repair or replace parts as needed.
17. Remove the wheel blocks