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DUAL
BRAKE
VALVE
TRAILER
SUPPLY
VALVE
™
FIGURE 4 - LQ-5
BOBTAIL RATIO VALVE LIMITING: TRACTOR-TRAILER MODE (SHOWN DURING INITIAL DELIVERY)
BALANCED BRAKE VALVE APPLICATION - LQ-5
VALVE HOLDING
The forces acting on the proportioning piston are equal when
air pressure beneath the piston is approximately one-half of
that above. (For example, if a 40 psi brake application is
made, approximately 20 psi would be present beneath the
proportioning piston.) At this point, the proportioning piston
lifts and closes the inlet. The exhaust passage remains
sealed. With both the inlet and exhaust valves closed, the
™
LQ-5
valve is balanced—front axle air pressure is neither
increasing nor decreasing.
BRAKE VALVE RELEASE
Upon brake valve release, air pressure above the blend back
piston returns to the brake valve and exhausts. Air pressure
from the delivery line lifts the proportioning piston and inlet/
exhaust valve away from the exhaust seat in the body. Air
then flows out the open exhaust passage. Air from the
actuators exhausts through the quick release valve. (See
Figure 2.)
If the service brakes are rapidly released, air pressure may
lift the inlet/exhaust valve further against the relatively light
resistance of the valve spring. Air would then also flow back
through the open inlet and out to the brake valve.
FRONT
BRAKE
CHAMBER
™
PROPORTIONING
INLET/
EXHAUST
VALVE
BRAKE VALVE APPLICATION
PRESSURE
™
FIGURE 5 - LQ-5
BOBTAIL RATIO VALVE HOLDING:
TRACTOR-TRAILER MODE
COVER
BLEND
BACK
SPRING
BODY
BLEND
BACK
PISTON
INITIAL
DELIVERY
PISTON
PISTON
APPROXIMATELY 1/2 OF
BRAKE VALVE
APPLICATION PRESSURE
3
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