bleeding and keep the metering valve from opening. In this
circumstance, it may not be possible to bleed the front
brakes. Therefore, many metering valves have an external
plunger which can be depressed during the bleeding oper-
ation to allow brake fluid to flow into the front wheel units.
Proportioning Valve
As mentioned earlier, when the brakes are applied,
much of the vehicle’s weight is transferred to the front
wheels. If the brakes are applied hard during an emergency
stop, so much vehicle weight is transferred to the front
wheels that the rear wheels can easily lose traction and
lock up. Rear wheel lockup can cause wear on the rear
tires and can cause the vehicle to spin out of control, espe-
cially on wet or icy roads. The danger of lockup is worse
on vehicles with rear drum brakes than on vehicles with
rear discs. However, rear wheel lockup is a problem on
many vehicles with four-wheel disc brakes, especially
those with front-wheel drive.
To prevent rear wheel lockup, a proportioning valve,
Figure 9-4, is installed in the rear brake line. Inside the pro-
portioning valve assembly, a calibrated spring holds the
valve away from the opening to the rear brakes.
Diagonally split braking systems may have two propor-
tioning valves. In some cases, the proportioning valves are
installed at the master cylinder outlets, Figure 9-5.
During normal braking, the system pressure cannot
overcome spring tension, and passes through the opening
to the rear brakes. When the brake pressure rises past a
certain point, such as during a panic stop, it will overcome
spring tension and push the valve against the opening. This
seals the rear brake system from further increases in pres-
sure. Since the amount of pressure entering the rear brakes
is limited, the brake friction elements cannot be applied
any harder. Note that rear brake pressure is not decreased,
Chapter 9 Hydraulic Valves, Switches, Lines, and Hoses 153
Figure 9-3. r A special metering valve installed in the master
cylinder body. (ATE)
Valve
cylinder Master s
body
Metering check
valve assembly
Figure 9-4. A dual proportioning valve setup. Note that with this
valve, a separate line is sent to each individual rear brake
assembly. (Bendix)
To right
front brake
Master cylinder
Secondary brake line
Primary brake line
To left front brake
Brake
pedal
Vacuum booster
To right rear brake
To left rear brake Dual
proportioning
valve
Figure 9 Figure 9 -5. 5. Proportioning valves which are connected to the master cylinder fluid outlet ports. (DaimlerChrysler) Proportioning valves which are connected to the master cylinder fluid outlet ports. (DaimlerChrysler)
Rear brake
hydraulic lines
Proportioning
val lv es
Master
cylinder
Front brake
hydraulic
lines
Master cylinder
brake fluid reservoir