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If the warning signal does not work, you could lose pressure and you would not know it. This could cause sudden emergency in a single-circuit air system. In dual systems, the stopping distance will be . 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 and the brake pedal to 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 brakes to come on.
Check Rate of Air Pressure Buildup. When the engine is at operating rpms, the pressure should build from to psi within seconds in dual air systems. (If the vehicle has larger than minimum air tanks, the buildup time can be and still be safe. Check the manufacturer’s specifications.) In single air systems (pre-1975), typical requirements are pressure buildup from to psi within 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 , requiring an emergency stop. Do not until you get the problem fixed.
Test Air Leakage Rate. With a fully-charged air system (typically psi), turn off the engine, the parking brake (push in) and the air pressure drop. The loss rate should be than psi in minute for single vehicles and than psi in minute for combination vehicles. With the air pressure built up to governor cut-off (120-140 psi), shut off the engine, chock your wheels (if necessary), the parking brake (all vehicles) and the tractor parking valve (combination vehicle), and fully apply the brake. Hold the brake for minute. Check the air gauge to see if the air pressure drops than pounds in minute (single vehicle) or pounds in minute (combination vehicle). If the air pressure falls than psi in minute for single vehicles ( than psi for combination vehicles), the air loss rate is too much. Check for air leaks and fix before driving the vehicle. Otherwise, you could your brakes while driving.
Check Air Compressor Governor Cut-in and Cut-out Pressures. Pumping by the air compressor should start at about psi and stop at about psi. (Check manufacturer’s specifications.) Run the engine at a idle. The air governor should cut-out the air compressor at about the manufacturer’s specified pressure. The air pressure shown by your gauge(s) will stop rising. With the engine idling, step on and off the brake to reduce the air tank pressure. The compressor should cut-in at about the manufacturer’s specified cut-in pressure. The pressure should begin to rise.
If the air governor does not work as described above, it may need to be fixed. A that does not work properly may not keep enough air pressure for safe driving.
Test Parking Brake. Stop the vehicle, put the parking brake on and gently against it in a gear to test that the parking brake will hold.
Test Service Brakes. Wait for air pressure, the parking brake, move the vehicle forward slowly (about mph) and apply the brakes firmly using the brake pedal. Note any vehicle “pulling” to one side, unusual feel or delayed stopping action. This test may show you problems, which you otherwise would not know about until you needed the brakes on the road.
Your score is sent to your instructor. Tip: both forms of a fraction are accepted (for example, “1/4” or “one-quarter”).