TRUCK SERVICE MANUAL
BRAKES-AIR
CHAPTER IX
SPRING BRAKE AND TWO-WAY CONTROL VALVE
SPRING BRAKE CONTROL VALVE
automatic application will result when both primary and
(MIDLAND ROSS)
secondary systems have depleted air supply to the 28-35
psi.
DESCRIPTION
Refer to Fig. 2 for following operational description.
The spring brake control valve is a manually operated
Fig. 2 illustrates valve in applied (out) position.
valve of push-pull type. When the valve is pushed "in"
the valve is open to supply air. The "out" position closes
Pushing piston in moves valve assembly off body
off supply air and allows delivered air to exhaust to
seat until contacting end cap seat. Air pressure at inlet
atmosphere, applying the spring brakes. Valve also
has free passage to outlet and is blocked from
features a plunger pressure sensing arrangement which
exhausting to atmosphere.
provides automatic return to "out" position when brake
system air pressure approaches low energy levels.
Pulling piston out moves valve assembly away from
end cap seat and contacts body seat. Air pressure at
inlet is blocked from entering either outlet or exhaust
passages. Air pressure in outlet has free passage to
exhaust to atmosphere.
The spring in the valve assists to move piston out
automatically if inlet pressure drops and effective
opposing force across the valve seat is overcome.
Fig. 1. Spring Brake Control Valve
OPERATION
The spring brake control valve serves to app and release
the spring actuated parking brakes The valve is in the
"in" position under normal operation on either a straight
Fig. 2. Cross Section of Spring Brake Control Valve
truck or tractor-trailer. Air supply passes through the
valve delivering system pressure to retract and hold
MAINTENANCE
parking brakes in released position. Manual pull "out"
closes off air supply and vents delivered air to
Once each year or every 100, 000 miles the valve
atmosphere, applying the parking brakes.
should be removed, disassembled and a repair kit
installed.
When valve is pushed "in" it will remain in this position as
long as supply line is 45 psi o0 more. If air pressure
SERVICE CHECKS
should drop to 28-35 psi during normal operation the air
control valve will automatically apply, moving "out." The
Leakage Test
Use air pressure source equipped with in-line manual
shut-off valve, air gauge known to be accurate and
connection. Connect air source to
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