Operating Instructions Using the Brake System
Model 330 PB1318 3/01
– 85 –
The wet tank receives air from the air dryer and cools it
somewhat, allowing moisture to condense for draining. Rela-
tively dry air is then supplied to the two service tanks for dis-
tribution to their respective brake circuits. The service tanks
are isolated from each other by check valves.
Dual Service Brake Treadle Valve: delivers air to the two
service brake circuits.
Double Check Valve: directs the higher air pressure from
either the rear (primary) or front (secondary) service tank to
the modulating valve.
Modulating Valve (SR–1): used only on full trucks, not trac-
tors. It performs four functions:
• Limits spring brake hold-off air pressure delivered to the
spring brake chambers.
• Provides a quick release of air pressure from the spring
brake chambers to speed spring brake application.
• Modulates spring brake application in proportion to front
service application in the event of a rear service failure.
• Prevents compounding of service and spring applica-
tions.
Relay valve (full truck): speeds up the application of the
rear service brake. It also incorporates a quick–release fea-
ture.
Brake Proportioning (BP-R1) valve (tractor): this valve
delivers air to the tractor's service brake chambers. It auto-
matically proportions the drive axle brake app
lication pressure when driving bobtail (without a trailer con-
nected). Proportioning the brakes allows full use of the steer
axle brakes and reduces lockup of the drive axle brakes. The
bobtail brake proportioning system is automatically turned off
when a trailer is connected.
Quick–Release Valve:
speeds the release of air from the
brake chambers. When air is released, the air in the brake
chambers is exhausted at the quick-release valve, rather
than exhausting back through the treadle valve.
Single Check Valve: allows air flow in one direction only.
Parking Brake Valve: yellow diamond-shaped knob. It con-
trols the application and release of the parking (spring)
brakes of truck or tractor-trailer combinations or of the tractor
alone. If the air system is being charged from zero pressure,
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