1995 JEEP YJ light

produce a condition similar to grab as the tire loses
and recovers traction.
Flat-spotted tires can cause vibration and wheel
tramp and generate shudder during brake operation.
A tire with internal damage such as a severe bruise
or ply separation can cause vibration and pull. The
pull will be magnified when braking.
DIAGNOSING PARKING BRAKE MALFUNCTIONS
Adjustment Mechanism
Parking brake adjustment is controlled by a ca-
ble tensioner mechanism. The cable tensioner,
once adjusted at the factory, will not need further
attention under normal circumstances. There are
only two instances when adjustment is required.
The first is when a new tensioner, or cables have
been installed. And the second, is when the ten-
sioner and cables are disconnected for access to
other brake components.
Parking Brake Switch And Warning Light Illumination
The parking brake switch on the lever, or foot
pedal, is in circuit with the red warning light. The
switch will illuminate the red light only when the
parking brakes are applied. If the light remains on
after parking brake release, the switch or wires are
faulty, or cable tensioner adjustment is incorrect.
If the red light comes on while the vehicle is in mo-
tion and brake pedal height decreases, a fault has oc-
curred in the front or rear brake hydraulic system.
Parking Brake problem Causes
In most cases, the actual cause of an improperly
functioning parking brake (too loose/too tight/wont
hold), can be traced to a drum brake component.
The leading cause of improper parking brake
operation, is excessive clearance between the
brakeshoes and the drum surface. Excessive
clearance is a result of: lining and/or drum
wear; oversize drums; or inoperative shoe ad-
juster components.
Excessive parking brake lever travel (sometimes de-
scribed as a loose lever or too loose condition), is the re-
sult of worn brakeshoes/drums, improper brakeshoe
adjustment, or incorrectly assembled brake parts.
A ``too loose'' condition can also be caused by inop-
erative brakeshoe adjusters. If the adjusters are mis-
assembled, they will not function. In addition, since
the adjuster mechanism only works during reverse
stops, it is important that complete stops be made.
The adjuster mechanism does not operate when roll-
ing stops are made in reverse. The vehicle must be
brought to a complete halt before the adjuster lever
will turn the adjuster screw.
A condition where the parking brakes do not hold, will
most probably be due to a wheel brake component.
Items to look for when diagnosing a parking brake
problem, are:
²rear brakeshoe wear or adjuster problem
²rear brake drum wear
²brake drums machined beyond allowable diameter
(oversize)
²parking brake front cable not secured to lever
²parking brake rear cable seized
²parking brake strut reversed
²parking brake strut not seated in both shoes
²parking brake lever not seated in secondary shoe
²parking brake lever or brakeshoe bind on support
plate
²brakeshoes reversed
²adjuster screws seized
²adjuster screws reversed
²holddown or return springs misassembled or lack
tension
²wheel cylinder pistons seized
Brake drums that are machined oversize are diffi-
cult to identify without inspection. If oversize drums
are suspected, diameter of the braking surface will
have to be checked with an accurate drum gauge.
Oversize drums will cause low brake pedal and lack
of parking brake holding ability.
Improper parking brake strut and lever installation
will result in unsatisfactory parking brake operation.
Intermixing the adjuster screws will cause drag, bind
and pull along with poor parking brake operation.
Parking brake adjustment and parts replacement pro-
cedures are described in the Parking Brake section.
MASTER CYLINDER/POWER BOOSTER TEST
(1) Start engine and check booster vacuum hose
connections. Hissing noise indicates vacuum leak.
Correct any vacuum leak before proceeding.
(2) Stop engine and shift transmission into Neu-
tral.
(3) Pump brake pedal until all vacuum reserve in
booster is depleted.
(4) Press and hold brake pedal under light foot
pressure.
(a) If pedal holds firm, proceed to step (5).
(b) If pedal does not hold firm and falls away,
master cylinder is faulty due to internal leakage.
Overhaul or replace cylinder.
(5) Start engine and note pedal action.
(a) If pedal falls away slightly under light foot
pressure then holds firm, proceed to step (6).
(b) If no pedal action is discernible, or hard pedal
is noted, power booster or vacuum check valve is
faulty. Install known good check valve and repeat
steps (2) through (5).
(6) Rebuild booster vacuum reserve as follows: Re-
lease brake pedal. Increase engine speed to 1500
rpm, close throttle and immediately turn off ignition.
5 - 8 SERVICE BRAKE DIAGNOSISJ

MASTER CYLINDERÐCOMBINATION VALVE
INDEX
page page
Combination Valve Replacement (Non-ABS)..... 16
General Service Information................. 15
Master Cylinder and Combination Valve Installation
(With ABS)............................. 20
Master Cylinder and Combination Valve Removal
(With ABS)............................. 17Master Cylinder Bench Bleeding.............. 21
Master Cylinder Installation (Non-ABS)......... 16
Master Cylinder Overhaul (4-Cylinder Models).... 16
Master Cylinder Removal (Non-ABS)........... 15
Reservoir Replacement (2-Piece Master Cylinder) . 19
GENERAL SERVICE INFORMATION
Master Cylinder
Two different master cylinders are used. A one-piece
cast aluminum cylinder is used on 4-cylinder YJ models
(Fig. 1). All other models have a two-piece master cylin-
der with removable nylon reservoir (Fig. 2).
The two master cylinders are serviced differently.
The reservoir and grommets are the only replaceable
parts on the two-piece master cylinder. The one-piece
master cylinder can be overhauled when necessary.
Combination Valve
A combination valve is used in all models. The
valve contains a pressure differential valve and
switch and a rear brake proportioning valve. The
valve is not repairable. It must be replaced if diagno-
sis indicates this is necessary.
The pressure differential switch is connected to the
brake warning light. The switch is actuated by move-
ment of the switch valve. The switch monitors fluid
pressure in the separate front/rear brake hydraulic cir-
cuits.
A decrease or loss of fluid pressure in either hydraulic
circuit will cause the switch valve to shuttle to the low
pressure side. Movement of the valve pushes the switch
plunger upward. This action closes the switch internal
contacts completing the electrical circuit to the red
warning light. The switch valve will remain in an actu-
ated position until repairs are made.
The rear proportioning valve is used to balance front-
rear brake action. The valve allows normal fluid flow
during moderate effort brake stops. The valve only con-
trols (meters) fluid flow during high effort brake stops.
MASTER CYLINDER REMOVAL (NON-ABS)
(1) Remove air cleaner hose, cover and housing.
Fig. 1 Master Cylinder And Combination Valve
(4-Cyl. YJ Models)
Fig. 2 Master Cylinder And Combination Valve (All
Except 4-Cyl. YJ Models)
JMASTER CYLINDERÐCOMBINATION VALVE 5 - 15

(2) Disconnect brake lines at master cylinder and
combination valve.
(3) Remove nuts attaching master cylinder to
booster studs.
(4) Remove master cylinder.
(5) Remove cylinder cover and drain fluid.
(6) If two-piece master cylinder reservoir requires
service, refer to reservoir replacement procedure in
this section.
MASTER CYLINDER INSTALLATION (NON-ABS)
(1) Bleed master cylinder on bench before installa-
tion. Refer to procedure in this section.
(2) If new two-piece master cylinder is being in-
stalled, remove plastic protective sleeve from primary
piston shank. Also check condition of seal at rear of
cylinder body. Reposition seal if dislodged. Replace
seal if cut, or torn.
(3) Clean cylinder mounting surface of brake booster.
Use shop towel wetted with brake cleaner for this pur-
pose. Dirt, grease, or similar materials will prevent
proper cylinder seating and could result in vacuum leak.
(4) Slide master cylinder onto brake booster studs.
(5) Install nuts attaching master cylinder to booster
studs. Tighten nuts to 25 Nzm (220 in. lbs.) torque.
(6) Connect brakelines to master cylinder and com-
bination valve (Figs. 1 and 2).
(7) Fill and bleed brake system.
COMBINATION VALVE REPLACEMENT (NON-ABS)
The combination valve is not a repairable compo-
nent. The valve is serviced as an assembly whenever
diagnosis indicates replacement is necessary.
(1) Remove air cleaner cover and hose for access to
valve, if necessary.
(2) Disconnect differential pressure switch wire at
combination valve. Do not pull switch wire to discon-
nect. Unsnap connecter lock tabs to remove.
(3) Disconnect brakelines at combination valve and
remove valve.
(4) Connect brakelines to replacement valve. Start
line fittings by hand to avoid cross threading.
Tighten fittings snug but not to required torque at
this time.
(5) Connect wire to pressure differential switch.
(6) Bleed brakes.
(7) Tighten brakeline fittings to 18-24 Nzm
(160-210 in. lbs.) torque after bleeding.
MASTER CYLINDER OVERHAUL (4-CYLINDER
MODELS)
CYLINDER DISASSEMBLY
(1) Examine cylinder cover seal. Discard seal if
torn or distorted.
(2) Clamp cylinder in vise (Fig. 3).(3) Remove piston retaining snap ring. Press and
hold primary piston inward with wood dowel or sim-
ilar tool. Then remove snap ring (Fig. 4).
(4) Remove and discard primary piston (Fig. 5).
Piston is serviced only as assembly.
(5) Remove secondary piston (Fig. 6). Apply air
pressure through rear outlet port to ease piston out
of bore. Cover small ports at bottom of rear reservoir
with towel to prevent air leakage.
(6) Discard secondary piston. Do not disassemble
piston as components are only serviced as assembly.
MASTER CYLINDER CLEANING AND
INSPECTION
Clean the cylinder with Mopar brake cleaning sol-
vent or clean brake fluid. Remove cleaning residue
with compressed air.
Inspect the cylinder bore. A light discoloration of
Fig. 3 Mounting Cylinder In Vise
Fig. 4 Removing/Installing Piston Snap Ring
5 - 16 MASTER CYLINDERÐCOMBINATION VALVEJ

POWER BRAKE BOOSTERÐBRAKE PEDALÐBRAKELIGHT SWITCH
INDEX
page page
Brake Pedal Installation..................... 31
Brake Pedal Removal...................... 31
Brakelight Switch Adjustment................. 32
Brakelight Switch Installation................. 32
Brakelight Switch Removal.................. 32
General Information....................... 22
Power Brake Booster Installation (XJ with ABS) . . . 27
Power Brake Booster Installation (XJ Without ABS).. 30
Power Brake Booster Installation (YJ).......... 30
Power Brake Booster Operation.............. 23
Power Brake Booster Removal (XJ with ABS).... 24
Power Brake Booster Removal (XJ Without ABS) . . 29
Power Brake Booster Removal (YJ)............ 30
GENERAL INFORMATION
A 205 mm (8.07 in.) dual diaphragm power brake
booster is used for all applications (Figs. 1 and 2).
The only serviceable parts on the power brake
booster (Figs. 1 and 2) are the check valve, and vac-
uum hose. The booster itself is not serviceable. Re-
place the booster as an assembly whenever diagnosis
indicates a malfunction has occurred.
Brake Pedal And Brakelight Switch
A suspended-type brake pedal is used on all mod-
els. The pedal pivots on a shaft mounted in the pedalsupport bracket. The bracket is attached to the dash
and instrument panels on all models.
A plunger-type, adjustable brakelight switch is
used on all models. The switch is attached to a flange
on the pedal support bracket.
The brake pedal is a serviceable component. The
pedal, pivot pin, sleeve, pedal bushings and spacers/
washers are all replaceable parts. The pedal bracket
can also be replaced when necessary.
Fig. 1 Brake Booster/Master Cylinder Assembly (XJ)
5 - 22 POWER BRAKE BOOSTERÐBRAKE PEDALÐBRAKELIGHT SWITCHJ

POWER BRAKE BOOSTER OPERATION
Booster Components
The booster assembly consists of a housing divided
into separate chambers by two internal diaphragms
(Fig. 2). The outer edge of each diaphragm is at-
tached to the booster housing. The diaphragms are
connected to the booster primary push rod.
Two push rods are used in the booster. The primary
push rod connects the booster to the brake pedal. The
secondary push rod connects the booster to the mas-
ter cylinder to stroke the cylinder pistons.
The atmospheric inlet valve is opened and closed
by the primary push rod. Booster vacuum supply is
through a hose attached to an intake manifold fitting
at one end and to the booster check valve at the
other. The vacuum check valve in the booster housing
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 a filter and inlet valve at the rear of the
housing (Fig. 3).
The chamber areas forward of the booster dia-
phragms are exposed to vacuum from the intake
manifold. The chamber areas to the rear of the dia-
phragms, are exposed to normal atmospheric pres-
sure of 101.3 kilopascals (14.7 pounds/square in.).
Brake pedal application causes the primary push
rod to open the atmospheric inlet valve. This exposes
the area behind the diaphragms to atmospheric pres-
sure. The resulting pressure differential provides the
extra apply pressure for power assist.
Fig. 2 Brake Booster/Master Cylinder Assembly (YJ)
JPOWER BRAKE BOOSTERÐBRAKE PEDALÐBRAKELIGHT SWITCH 5 - 23

POWER BRAKE BOOSTER REMOVAL
(XJ WITH ABS)
(1) Disconnect vacuum and vent hoses at air
cleaner cover.
(2) Loosen clamp securing air cleaner hose to in-
take manifold. Use screwdriver to tap clamp loose.
(3) Remove air cleaner cover and hose. Then re-
move air filter from air cleaner housing (Fig. 4).
(4) Remove two bolts and one nut that secure air
cleaner housing to body (Fig. 4).
(5) Remove air cleaner housing from engine com-
partment (Fig. 4).
(6) Disconnect wire at combination valve pressure
differential switch (Fig. 5). Do not pull on wires to
disconnect. Unsnap lock tabs on connecter to remove
wires.
(7) Disconnect canister vacuum line at manifold
fitting (Fig. 6).(8) Disconnect brake booster vacuum hose at in-
take manifold fitting (Fig. 7). Move hose aside for
working clearance.
(9) Unseat small S-clip that secures brakelines
(Fig. 8).
(10) Remove front brakeline that connects master
cylinder front port to combination valve front port
(Fig. 8).
Fig. 3 Power Brake Booster Internal Components
5 - 24 POWER BRAKE BOOSTERÐBRAKE PEDALÐBRAKELIGHT SWITCHJ

Fig. 4 Air Cleaner Components
Fig. 5 Pressure Differential Switch Wire Connection
Fig. 6 Canister Vacuum Line Location
(At Manifold Fitting)
Fig. 7 Booster Vacuum Hose Removal/Installation
(From Manifold Fitting)
Fig. 8 Master Cylinder Front Brakeline Removal/
Installation
JPOWER BRAKE BOOSTERÐBRAKE PEDALÐBRAKELIGHT SWITCH 5 - 25

(11) Disconnect master cylinder rear brakeline at
cylinder. Then loosen line at combination valve and
swing line around to opposite side of cylinder (Fig. 9).
(12) Disconnect rear brakeline at HCU (Fig. 10).
(13) Disconnect both flex brakelines at HCU (Fig.
10).
(14) Disconnect HCU line to rear brakes at HCU
port (Fig. 10).
(15) Remove nut attaching combination valve
bracket to brake booster stud.
(16) Remove combination valve and brakelines as
assembly (Fig. 11). Work valve bracket off booster
stud. Then work brakelines around cylinder and
HCU and remove assembly.
(17) Remove nuts attaching master cylinder to
booster studs and remove cylinder (Fig. 12).
(18) Remove master cylinder reservoir cap and
drain fluid.
(19) Disconnect HCU solenoid harness from main
harness (Fig. 13).
(20) Disconnect HCU pump motor harness (Fig.
14).(21) Disconnect lines at lower left side of HCU
(Fig. 15).
(22) Remove nuts attaching HCU mounting
bracket to stud plate and body. Then remove HCU
and bracket as assembly.
(23) In passenger compartment, remove instru-
ment panel lower trim cover.
(24) Remove retaining clip that secures booster
push rod to brake pedal (Fig. 16).
(25) Remove nuts attaching booster to passenger
compartment side of dash panel.
(26) In engine compartment, slide booster studs
out of dash panel, tilt booster upward, and remove
booster from engine compartment.
(27) Remove booster spacer, if equipped.
(28) Remove dash seal from booster, or dash panel.
Fig. 9 Disconnecting Master Cylinder Rear Brakeline
Fig. 10 Location Of HCU Flexlines And HCU Line To
Rear Brakes
Fig. 11 Combination Valve And Brakeline Removal
Fig. 12 Master Cylinder Attaching Nut Removal
5 - 26 POWER BRAKE BOOSTERÐBRAKE PEDALÐBRAKELIGHT SWITCHJ