AIR BRACKERS-GENERAL INFORMATION
Figure 21 - Power Divider Lock Air Shift Control With
Separate Spring Brake and Tractor Protection Control
Valves
BRAKE AIR PRESSURE DISTRIBUTION VALVE
Prior to the introduction of FMVSS 121 (March, 1975),
front wheel limiting valves could be used on trucks and
buses with air brakes. This would permit full brake valve
delivery pressure to front wheel brakes when operating
on dry roads, or at the option of the operator, reduce the
pressure delivered to the front wheel brakes by 50%
when operating on wet or slippery roads.
Since the introduction of FMVSS 121, Federal Highway
Administration,
Department
of
Transportation,
Subchapter B, Federal Motor Carrier Safety Regulation
states that, "Manually operated devices to reduce or
remove the front wheel braking effort must not be
installed in a bus, truck or truck-tractor manufactured
after February 28, 1975."
The regulation, however, does permit an automatic type
device to reduce front wheel braking up to 50% of normal
braking with certain specific limitations. It should be
noted that this is a constant function and is not
controllable by the driver.
There is a valve made available (code 04567) for special
requests. It, however, is not a true front wheel limiting
valve but a brake air pressure distribution valve. This
valve only limits full front wheel braking during light brake
applications regardless of road conditions. However, in a
hard or panic brake application, the driver obtains full
front wheel braking (full braking potential).
Figure 22 - Power Divider Lock Air Shift Control With
Modular Control Valve For Spring Brakes and Tractor
Protection Control Valve
There are other considerations which one should be
aware of with the installation of the air pressure
distribution valve. They are:
Increased Rear Axle Brake Wear
The vehicle’s brake wear balance, as designed, Is equal
on front and rear brakes. During normal brake stops
with the installation of an air pressure distribution valve,
air pressure to front axle brakes is half of that applied to
rear brakes, resulting in abnormal wear of rear service
brake linings.
Glazing and Noise
During continued light brake applications glazing, poor
burnish and noise problems could increase. The air
pressure distribution valve reduces front axle braking
work by about half.
CTS-4154Y - Page 19
