Figure 7-19.Fixed multipiston caliper.
The stoplight switch is a spring-loaded electrical
switch that operates the rear brake lights of the vehicle.
Most modern vehicles use a mechanical switch on the
brake pedal mechanism. The switch is normally open,
and when the brake pedal is depressed, the switch
closes and turns on the brake lights.
O n s o m e o l d e r v e h i c l e s y o u m a y f i n d
hydraulically operated stoplight switches. In this
system, brake pressure acts on a switch diaphragm,
which closes the switch to turn on the brake lights.
Brake light circuits are covered in chapter
2 of this TRAMAN.
Brake Warning Light Switch
The brake Learning light switch, also called the
pressure differential valve, warns the operator of a
pressure loss on one side of a dual brake system. If a
leak develops in either the primary or secondary brake
system. unequal pressure acts on each side of the
warning light piston, moving the piston to one side
thereby grounding the switch.
The metering valve is designed to equalize braking
action at each wheel during light brake applications. A
metering valve is used on vehicles with front disc
brakes and rear drum brakes and is located in the line to
the disc brakes. The metering valve functions by
preventing the disc brakes from applying until
approximately 75 to 135 psi has built up in the system.
The proportioning valve also equalizes braking
action with front disc brakes and rear drum brakes. It is
located in the brake line to the rear brakes. The
function of the proportioning valve is to limit pressure
to the rear brakes when high pressure is required to
apply the front disc. This prevents rear wheel lockup
and skidding during heavy brake applications.
The combination valve (fig. 7-20) combines
several valve functions into a single assembly. It
functions as a
Metering valveholds off front disc braking
until the rear drum brakes make contact with the
Proportioning valveimproves front to rear
brake balance at high deceleration by reducing
rear brake pressure to delay rear wheel skid.
Brake light warning switch (pressure differential
valve)lights a dash-warning lamp if either
front or rear brake systems fail.
ANTILOCK BRAKE SYSTEM (ABS)
The antilock brake system (ABS) is used because
it provides CONTROL. Skidding causes a high
percentage ofvehicle accidents on the highway and the
ABS (fig. 7-21), also known as a skid control brake
system, uses wheel speed sensors, hydraulic valves,
and the on-board computer to prevent or limit tire