Note: Your are not logged in. We can not keep your scores or track your progress unless you Register and Log In

5.3.3 – Step 7 Final Air Brake Check

Do the following checks instead of the hydraulic brake check shown in "Section 2, Step 7: Check Brake System."

Test Low Pressure Warning Signal. Shut the engine off when you have enough air pressure so that the low-pressure warning signal is not on. Turn the electrical power on and step on and off the brake pedal to reduce air tank pressure. The low air pressure warning signal must come on before the pressure drops to less than 60 psi in the air tank (or tank with the lowest air pressure, in dual air systems).

If the warning signal does not work, you could lose air pressure and you would not know it. This could cause sudden emergency braking in a single-circuit air system. In dual systems, the stopping distance will be increased. Only limited braking can be done before the spring brakes come on.

Check That Spring Brakes Come On Automatically. Continue to fan off the air pressure by stepping on and off the brake pedal to reduce tank pressure. The tractor parking valve and parking brake valve should close (pop out) on a tractor-trailer combination vehicle, and the parking brake valve should close (pop out) on other combination and single vehicle types when the air pressure falls to the manufacturer’s specification (20-45 psi). This will cause the spring brakes to come on.

Check Rate of Air Pressure Buildup. When the engine is at operating RPMs, the pressure should build from 85 to 100 psi within 45 seconds in dual air systems. (If the vehicle has larger than minimum air tanks, the buildup time can be longer and still be safe. Check the manufacturer's specifications.) In single air systems (pre-1975), typical requirements are pressure buildup from 50 to 90 psi within three minutes with the engine at an idle speed of 600-900 RPMs.

If air pressure does not build up fast enough, your pressure may drop too low during driving, requiring an emergency stop. Do not drive until you get the problem fixed.