1-17. BRAKE SYSTEM. - Continued
(5) If the primary system becomes depleted of air, the secondary system is indirectly used to apply the rear
brakes. The shift from primary to secondary air is automatic and carried out in the spring brake valve when the brakes
(6) Normally, primary air pressure, supplied via the relay valve, keeps the secondary (control) air supply port in
the spring brake valve closed. This action keeps the valve function neutral.
(7) With no primary air available, secondary air pressure causes the spring brake to partially close and reduce
the air pressure that keeps the parking brakes disengaged, (see fig. 1-32). When this occurs, the quick release valve
automatically opens and momentarily vents the air chambers to the atmosphere. The reduction in pressure in the air
chambers is proportional to the secondary air pressure applied to the spring brake valve.
Figure 1-32. Rear Brake System - Secondary Air Only