1
®
SD-03-952
Bendix
®
BP-1
™
Brake Proportioning Valves
FIGURE 1
BP-1
™
BRAKE PROPORTIONING VALVES
1.5
.340”
MOUNTING
HOLES (2)
I.D. TAG STAMPED
BP-1 FRONT OR
BP-1 REAR
3/8 P.T.
SUPPLY PORT
3/8 P.T.
DELIVERY PORT
1/8 P.T.
CONTROL PORT
PIECE NO.
STAMPED HERE
DESCRIPTION OF THE BP-1
™
VALVES
The BP-1
™
brake proportioning valves
are incorporated into
the air brake systems to improve the controllability and
reduce the stopping distance of bobtail tractors during
braking. BP-1
™
, front and rear, valves reacting to the lack of
pressure in the trailer supply line reduce the braking effort
on the rear axles and increase the braking effort on the front
axle of tractors operating in the bobtail mode. Treadle
application force during bobtail operation resembles treadle
application force during normal operation with a connected
trailer.
A typical brake proportioning system is shown in
Figure
6.
The system consists of a front and a rear brake proportioning
valve and a TR-3
™
valve.
Caution:
The BP-1
™
front and BP-1
™
rear valves are similar
in appearance but are NOT interchangeable. Note
identification tag attached to one of the cover
screws (Figure 1); it will be stamped ‘BP-1 Front’
or ‘BP-1 Rear’. The BP-1
™
rear valve can be used
without installing a BP-1
™
front valve, providing the
vehicle does not have a front axle ratio valve. If the
vehicle has a front axle ratio valve, the ratio valve
must be replaced with a BP-1
™
front valve.
OPERATION
A tractor that is equipped with the BP-1
™
rear brake
proportioning valve and TR-3
™
valve only will respond in the
following manner. With the trailer supply valve pushed in
and a service brake application made, equal pressure (100%
of service application) will be delivered to each axle of the
tractor and trailer. When the trailer is disconnected from the
tractor and the trailer supply valve is pulled out, the front
axle of the tractor will receive 100% of the service brake
application pressure and the rear axle or axles approximately
25%. As application pressure increases, the differences
between the front axle and the rear axle(s) pressures
diminish.
