Bendix Commercial Vehicle Systems LQ-5 BOBTAIL RATIO VALVE 4/04 - Manual

1
®
SD-03-953
Bendix ® LQ-5 ™ Bobtail Ratio Valve
Although it is not mandatory, the LQ-5 ™ valve is used in
conjunction with the BP-R1 ™ bobtail brake proportioning relay
valve, only when a customer specifies a front axle ratio valve.
The BP-R1
™ valve and LQ-5 ™ valve combine to reduce
stopping distance and improve tractor control in bobtail mode.
(For more information on the BP-R1
™ valve, see SD-03-1067.)
OPERATION
GENERAL
After an initial delivery, the LQ-5 ™ valve limits front axle brake
pressure during normal tractor-trailer operation. The LQ-5 ™
valve has two spring-loaded pistons: a blend back piston
and a proportioning piston. The “stepped” proportioning piston
determines actual air pressure reduction and the blend back
piston allows the valve’s delivery pressure to “blend back”
and reach a 1:1 ratio during high pressure brake applications.
DESCRIPTION
The LQ-5 ™ bobtail ratio valve is used on the front (steering)
axle of tractor air brake systems to reduce brake applica-
tion pressure during normal tractor-trailer operation. It is
designed for tractor systems only and replaces the existing
front axle limiting valve. It is not to be used on straight trucks
or towing trucks. Also, a quick release valve must be in-
stalled between the LQ-5
™ valve delivery and the brake
chambers.
IMPORTANT: Do not replace the BP-1 ™ front brake pro-
portioning valve with the LQ-5 ™ valve. The LQ-5 ™ valve
will not function properly in the BP-1 ™ valve system.
Caution: The LQ-5 ™ valve can replace the LQ-4 ™ ratio valve
on tractors. The LQ-5 ™ valve control port must be connected
to the trailer supply line. When the tractor is disconnected
from the trailer, bobtail tractor braking performance is improved
because full brake valve pressure will be applied to the
steering axle brakes.
FIGURE 1 - LQ-5 ™ BOBTAIL RATIO VALVE
3/8 P.T.
SUPPLY PORT
1/8 P.T.
CONTROL PORT
1.5"
.340”
MOUNTING
HOLES (2)
STAMPED LQ-5
REPLACEMENT
PC. NO.
3/8 P.T.
DELIVERY PORT