Troubleshooting guide

22
www.bendix.com 1-800-AIR-BRAKE (1-800-247-2725)
Bobtail Ratio Valves
The LQ-5
™
bobtail ratio valve is used on the front
(steering) axle of tractor air brake systems to reduce
brake application pressure during normal tractor-trailer
operation. During bobtail mode (when the tractor is
not pulling a trailer), tractor braking performance is
improved because the LQ-5
™
bobtail ratio valve delivers
full brake pressure to the steering axle.
The LQ-5
™
bobtail ratio valve is designed for tractor
systems only, and it replaces the existing front axle
limiting valve.
Ratio, Proportioning Valves
Air Brake Proportioning Valves
The BP-1
™
brake proportioning valves are
incorporated into the air systems to improve the
controllability and reduce the stopping distance of
bobtail operated tractors during braking. The TR-3
™
valve senses the lack of trailer supply line pressure
during bobtail operation, and controls the BP-1
™
FRONT and REAR valves. The BP-1
™
FRONT valve
reduces application pressure to the front brakes during
tractor trailer operation and returns to full application
pressure during bobtail operation. The BP-1
™
REAR
valve delivers full pressure during tractor trailer
operation and reduces application pressure during
botail operation. Treadle application force, during
bobtail operation, resembles treadle application force,
during tractor operation with a loaded trailer.
Both the BP-1
™
FRONT and REAR valve are identical
in appearance to the LQ-5
™
valve, however they are
not interchangeable. A metal tag identifies the BP-1
™
valve.
Brake Proportioning Relay Valves
The BP-R1
™
bobtail proportioning relay valve is
a combination of two valves in a single housing. The
lower portion contains a "standard" service brake relay
valve, which functions as a relay station to speed up
brake application and release. The upper portion
houses a brake proportioning valve which reduces
normal service brake application pressure when the
tractor is not towing a trailer (bobtail operation). The
control port on the BP-R1
™
valve is connected to the
trailer supply valve delivery and signals bobtail
operation.
BP-R1
™
Valve
BP-1
™
Valve
Tag Indicates Valve
Name and Usage
LQ-5
™
Bobtail Ratio
Valve
PP-7
™
Valve
LQ-5
™
Valve