it is merely kept from going any higher. When the brake
pedal is released, the fluid pressure drops, and the spring
opens the passage to the rear brakes. Fluid pressure can
then return to the master cylinder.
Load-sensing Proportioning Valve
A variation of the proportioning valve is the load-sensing
proportioning valve or height-sensing proportioning valve.
This type of valve can vary the maximum pressure to the
rear brakes according to the load placed on the vehicle.
The load-sensing proportioning valve can be thought of as
a standard proportioning valve with a mechanism that
controls rear brake pressure. Load-sensing proportioning
valves are used on pickup trucks, vans, and other vehicles
designed to regularly carry heavy loads. The valve is usu-
ally installed at the rear wheels, mounted on the vehicle
frame as shown in Figure 9-6. A link or lever from the
valve is connected from the rear axle assembly.
In operation, the link moves an adjusting mechanism
inside the proportioning valve assembly. See Figure 9-7.
Loading the vehicle causes the body to compress the
springs, which reduces the distance between the axle and
frame. The reduced distance between the axle and frame
causes the link to move the adjustment mechanism. This
increases spring tension on the proportioning valve, which
allows more pressure to enter the rear brakes before the
valve closes. Therefore, the rear brakes take more of the
braking effort when the vehicle is loaded.
154 Auto Brakes
Note: Do not confuse this valve with a
height sensor for a ride control suspension
system, they are not the same.
Figure 9 Figure 9 -6. 6. Overall view of one particular regulated (load sensing) proportioning valve, and its operating linkage. (GMC) Overall view of one particular regulated (load-sensing) proportioning valve and its operating linkage (GMC)
Bushing an nd
e linka ge
Vehicle frame Regulated Vehicle frame Regulated
Figure 9-7. A—Hydraulic pressure chart. B—Regulated
(load-sensing) proportioning valve and linkage assembly
depicting the linkage and valve changes in the original and
raised frame positions. (Bendix, Chevrolet)
Input hydraulic pressure (oil pressure
applied from master cylinder)
When fail-safe is