Chrysler RAM TRUCK 2500 1994 Service Manual page 175

5 - 1 6 POWER BRAKE BOOSTER—BRAKE PEDAL—BRAKELIGHT SWITCH
•
PEDAL SHAFT
BRAKELIGHT
SWITCH J9405-87
Fig. 3 Brakelight Switch Location And Mounting
POWER BRAKE BOOSTER SERWICEABILITY
The only serviceable power brake booster compo­
nents are the vacuum hose and check valve. The
booster itself is not a repairable component. The
booster must be replaced as an assembly whenever
diagnosis indicates a fault has occurred.
POWER BRAKE BOOSTER OPERATION
Booster Components
The booster assembly consists of a housing divided
into separate chambers by two internal diaphragms.
The outer edge of each diaphragm is attached to the
booster housing. The diaphragms are i n turn, con­
nected to the booster push rod.
Two push rods are used to operate the booster. One
push rod connects the booster to the brake pedal. The
second push rod (at the forward end of the housing),
strokes the master cylinder pistons. The rear push
rod is connected to the two diaphragms in the
booster housing.
The atmospheric inlet valve is opened and closed
by the push rod connected to the brake pedal. The
booster vacuum supply is through a hose attached to
a fitting on the intake manifold. The hose is con­
nected to a vacuum check valve i n the booster hous­
ing. The check valve is a one-way device that
prevents vacuum leak back.
How Brake Boost Is Generated
Power assist is generated by utilizing the pressure
differential between normal atmospheric pressure
and a vacuum. The vacuum needed for booster oper­
ation is taken directly from the engine intake mani­
fold. The entry point for atmospheric pressure is
through an inlet valve at the rear of the housing.
The forward portion of the booster housing (area i n
front of the two diaphragms), is exposed to manifold
vacuum. The rear portion (area behind the dia­
phragms), is exposed to normal atmospheric pressure
of 101.3 kilopascals (14.7 pounds/square in.).
Pressing the brake pedal causes the rear push rod
to open the inlet valve. This exposes the area behind
the diaphragms to atmospheric pressure. The result­
ing force applied to the diaphragms is what provides
the extra boost i n apply pressure for power assist.
POWER BRAKE BOOSTER REMOVAL
(1) Disconnect harness wires from differential
switch on combination valve and from RWAL valve.
(2) Disconnect brakeline from combination valve to
front brakes, or ABS valve. Then disconnect brake-
line from RWAL valve to rear brakes.
(3) Remove nuts attaching master cylinder and
valves to power brake booster studs. Then remove
master cylinder and valves as assembly (Fig. 4).
(4) Disconnect vacuum hose at booster check valve.
(5) Remove knee bolster for access to brake pedal
(Fig. 5).
(6) Remove clip and washer securing booster push
rod to brake pedal (Fig. 6). Then slide booster push
rod off pedal.
(7) Remove nuts attaching booster mounting studs
to dash panel and pedal mounting bracket (Fig. 7).
(8) Remove booster from engine compartment.
(9) I f booster will be replaced, note and record code
letters on front face of booster.