Medium Duty Operator's Manual - Peterbilt Motors Company

USING THE BRAKE SYSTEM
Brake Components
The following is a brief description of the air/brake system.
It is intended to supply you with general information on
how the system works. For complete information see the
Medium Duty Maintenance Manual.
Compressor: supplies air to the system. System pressure
is controlled by the governor.
Governor: controls the air pressure in the system by
actuating the compressor discharge mechanism. Its cutout
pressure is 115 to 125 psi (793 to 862 kPa). Its preset
cut-in pressure is set to between 13 to 25 psi (90 to 172
kPa) below the cut-out pressure setting (cut-out and cut-in
interval is not adjustable).
Safety Valve: installed on the supply reservoir outlet. It
should “vent off” at 150 psi (1034 kPa) permitting air to
escape.
Air Dryer (Option): collects and removes moisture and
contaminants from the air as it travels from the compressor
to the wet tank (reservoir).
STARTING AND OPERATING THE VEHICLE
Compressed Air Tanks: The wet (supply) and dry (service)
tanks are located behind the battery box and on the
frame rail; the supply tank is below the front service tank.
See “Air Tanks” on page 162.
The wet tank receives air from the air dryer and cools it
somewhat, allowing moisture to condense for draining.
Relatively dry air is then supplied to the two service tanks
for distribution 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
tractors. 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.
– 116 – Y53-6008A Medium Duty (R05/09)