If the air governor does not work as described  above, it may need to be fixed. A governor 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 pull against it in a low  gear to test that the parking brake will hold.  Test Service Brakes. Wait for normal air  pressure, release the parking brake, move the  vehicle forward slowly (about five 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 wouldn't know about until you needed  the brakes on the road.  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 protection 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 3 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. Don't drive until you get the  problem fixed.  Test Air Leakage Rate. With a fully-charged air  system (typically 125 psi), turn off the engine,  release the parking brake, and time the air  pressure drop. The loss rate should be less than  two psi in one minute for single vehicles and less  than three psi in one minute for combination  vehicles. Then apply 90 psi or more with the brake  pedal. After the initial pressure drop, if the air  pressure falls more than three psi in one minute for  single vehicles (more than four psi for combination  vehicles), the air loss rate is too much. Check for  air leaks and fix before driving the vehicle.  Otherwise, you could lose your brakes while  driving.  Check Air Compressor Governor Cut-in and  Cut-out Pressures. Pumping by the air  compressor should start at about 100 psi and stop  at about 125 psi. (Check manufacturer's  specifications.) Run the engine at a fast 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.  Figure 5.5  2005 Commercial Driver’s License Manual  Page 5-7  Section 5 – Air Brakes 