Pretrip Inspection and Daily Maintenance 39.1 Release the parking brakes on the vehicle by pushing in the parking brake control valve knob. See Fig. 9.7 . Check the air governor cut-in and cut-out pressures. Run the engine at fast idle. The air governor should cut out the air compressor at approximately 125 psi (862 kPa). With the engine idling, make a series of foot valve applications. The air gov- ernor should cut in the air compressor at approximately 95 psi (655 kPa). If the air governor does not cut in and out as de- scribed above, it must be adjusted to these specifications. If the air governor cannot be adjusted or repaired, replace it before oper- ating the vehicle.
Release the parking brakes on the vehicle by pushing in the parking brake control valve knob. With the air system fully charged to 125 psi (862 kPa), make one full brake ap- plication and note the air pressure reading on the gauge. Continue to reduce the air pressure by moderate brake applications to a maximum of 90 psi (620 kPa), then run the engine at governed rpm. If the time re- quired to raise the air pressure to 125 psi (862 kPa) (from the pressure noted after one brake application) is more than 30 seconds, eliminate any leaks, or replace the air com- pressor before operating the vehicle. 39.3 Check the air pressure reserve.
With the air system fully charged to 125 psi (862 kPa), stop the engine and note the air pressure. Then make one full brake application and observe the pressure drop. If it drops more than 25 psi (172 kPa), all areas of leakage must be eliminated before operating the ve- hicle. Fig. 9.7, Parking Brake Valve Knob f421624 39.4 Check the air leakage in the system. With the parking brakes applied, the trans- mission out of gear, and the tires chocked, charge the air system until cutoff pressure of 125 psi (862 kPa) is reached. 39.2 Check the air pressure buildup time as fol- lows: 9.21