CDL Air Brakes
When should the spring brakes pop out?
During the parking-brake control test, the control should normally pop out and apply the spring brakes while air pressure is in the 20-to-45 PSI range.
Recall this range
20 to 45 PSI
Continue reducing system pressure after the low-air warning. Observe the parking-brake control and call the pressure when it pops out. Use the procedure and acceptance criteria in your current state manual.
Keep the two checks separate
The low-air warning check asks when the warning signal activates. The spring-brake check continues as pressure falls and asks when the parking-brake control pops out. They are related, but they are not the same observation.
- Begin from the test's required charged-system setup.
- Reduce pressure with repeated service-brake applications.
- Call the low-air warning when it activates.
- Keep reducing pressure and observe the parking-brake control.
- Call the pop-out pressure and confirm it falls in the accepted range.
Official source
Check Section 5: Air Brakes.
Review the current California Commercial Driver Handbook, Section 5, then confirm the procedure in your state's CDL manual.