This chapter covers the valves and electrical switches
used to control and monitor the brake hydraulic system. It
also covers the steel and flexible hydraulic lines used to
connect the brake hydraulic devices, as well as fittings and
flares necessary for proper brake system operation. This
chapter also discusses the low fluid level and brake light
Pressure Control Valves and
All modern brake hydraulic systems contain pressure
control valves. These systems also contain brake switches,
which operate dashboard-mounted warning lights, or illu-
minate the rear brake lights. Pressure control valves affect
the hydraulic pressure delivered to the wheel units, which
can make brake action more efficient. Brake switches are
used to illuminate warning lights on the dashboard, alert-
ing the driver to the presence of a brake problem. Some
valves and switches work together to sense problems and
warn the driver. The following sections explain the con-
struction and operation of these valves and switches
If the front brakes apply before the rear brakes, vehi-
cle stability is affected. If the front wheels grip before the
rear wheels, the back of the vehicle will have more
momentum than the front. The back tries to pass the front,
which can cause the vehicle to become unstable, possibly
resulting in a skid.
Unfortunately, gravity and the design of modern brake
systems makes this problem worse. When the brakes are
first applied, much of the vehicle’s weight is transferred to
the front wheels. This gives the front wheels more traction,
and therefore, more stopping power than the rear wheels.
In addition, many modern vehicles are designed with front
disc and rear drum brakes.
Metering Valve
As you learned in Chapter 5, the front brake pads are
very close to the rotor and are not held in a retracted posi-
tion. When the brakes are first applied, hydraulic pressure
moves the front disc pads into contact with the rotor
almost immediately. However, the rear brake shoes are
held in the retracted position by return springs and must
overcome spring pressure to move the shoes into contact
with the drum. This means the front pads would apply
much more quickly than the rear shoes. A metering valve
is used to keep this from happening.
The metering valve, Figure 9-1, is installed in the brake
line between the master cylinder and the front brakes.
Vehicles with diagonally split braking systems will have two
metering valves. The basic metering valve design contains a
spring-loaded valve, which remains closed when no pres-
sure is applied. When the brake pedal is applied, master
cylinder pressure is sent to the front and rear wheels. The
rear wheels receive full pressure immediately. Since fluid
flows to the rear brakes immediately, system pressure
remains relatively low and the closed metering valve keeps
the front brakes from applying.
When the rear brakes have applied, there is no more
fluid movement into the rear hydraulic units, and system
pressure rises. When the system pressure reaches a certain
point, the metering valve opens, Figure 9-2. Full pressure
then applies the front brakes. When the brakes are
released, fluid can bypass the main metering valve and
return to the master cylinder. While most metering valves
are mounted in the brake lines or in a combination valve,
some metering valves are installed in the master cylinder,
Figure 9-3.
After the front brakes are serviced, the front brake
lines may contain air. This air may compress during brake
152 Auto Brakes
Figure 9-1. A cross-sectional view of a metering valve. Shown
with the valve closed; no fluid pressure is being transmitted to
the brake calipers. (Niehoff)
To front To front
e disc bra ke
To front
disc brake
Pressure from
master cylinder
Fluid press sure builds
until valve opens ve opens
Figure 9-2. A metering valve that is ready to open from
hydraulic pressure. When it opens, brake fluid under pressure
(arrows) will travel to the front brake calipers. (Raybestos)
To front
disc brake
To front
disc brake
Pressure from
master cylinder
Fluid pressure s builds
until valve opens ve opens
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