Normally the safety valve remains inoperative and only functions if reservoir pressure exceeds 150 psi (1034.3 kPa/10.3
bar). Constant exhausting of air from the safety valve can be caused by a faulty safety valve, governor, compressor
unloading mechanism, or a combination of any of these.
The primary reservoir provides the supply for the dual brake valve, the parking brake valve, the transmission shift valve,
and the R12 relay valve for the spring or parking brake release.
Pressure Protection Valves
Three pressure protection valves are used in this air system. Two of the pressure protection valves are set at 85 psi (586
kPa/5.86 bar) while the third is set at 75 psi (517.12 kPa/5.17 bar). The pressure protection valves are essentially check
valves. They open approximately 10 to 15 psi (68.95 to 103.42 kPa/.689 to 1.03 bar) above their closing pressure. The 75
psi (517.12 kPa/5.17 bar) pressure protection valve for example, will open at approximately 85 to 90 psi (586 to 620.55
kPa/5.86 to 6.2 bar) and will close when pressure drops to 75 psi (517.12 kPa/5.17 bar). These valves protect a circuit if a
line is ruptured and ensure a priority supply to the brakes.
Air entering the supply port is initially prevented from flowing out the delivery port by the inlet valve which is held closed by
the pressure regulating spring above the piston. When sufficient air pressure builds beneath the piston to overcome the
setting of the regulating spring, the piston will move, causing the inlet valve to unseat (open), and allow air to flow out the
delivery port. As long as air pressure at the supply port and beneath the piston remains above the specified closing
pressure, the inlet valve will remain open. Closing pressure is noted on the label affixed to the valve.
If for any reason system air pressure is decreased below the specified closing pressure, the regulating spring will move the
piston closing the inlet valve. The remaining air pressure at either the supply or delivery side, (depending upon where the
pressure drop has occurred) will be retained.
The spool of the air/transmission swivel is attached by setscrews to a spool mounting plate which is bolted to the spool of
the hydraulic swivel. This permits the air/transmission swivel spool to remain stationary with the chassis. The
air/transmission swivel barrel is attached to the hydraulic swivel barrel by means of a slotted arm which secures it to a
keying lug welded to the hydraulic swivel barrel. This permits both barrels to turn with the superstructure.