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5 - 10
BRAKES
DIAGNOSIS AND TESTING (Continued)
ferential switch and valve have been actuated due to
a leak in the hydraulic system.
On models with ABS brakes, the amber warning
lamp only illuminates during the self test and when
an ABS malfunction has occurred. The ABS lamp
operates independently of the red warning lamp.
For additional information refer to Group 8W.
MASTER CYLINDER/POWER BOOSTER
(1) Start engine and check booster vacuum hose
connections. A 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. The pedal should hold firm, if the pedal
falls away master cylinder is faulty (internal leak-
age).
(5) Start engine and note pedal action. It should
fall away slightly under light foot pressure then hold
firm. If no pedal action is discernible, power booster,
vacuum supply, or vacuum check valve is faulty. Pro-
ceed to the POWER BOOSTER VACUUM TEST.
(6) If the POWER BOOSTER VACUUM TEST
passes, rebuild booster vacuum reserve as follows:
Release brake pedal. Increase engine speed to 1500
rpm, close the throttle and immediately turn off igni-
tion to stop engine.
(7) Wait a minimum of 90 seconds and try brake
action again. Booster should provide two or more vac-
uum assisted pedal applications. If vacuum assist is
not provided, booster is faulty.
POWER BOOSTER VACUUM TEST
(1) Connect vacuum gauge to booster check valve
with short length of hose and T-fitting (Fig. 7).
(2) Start and run engine at curb idle speed for one
minute.
(3) Observe the vacuum supply. If vacuum supply
is not adequate, repair vacuum supply.
(4) Clamp hose shut between vacuum source and
check valve.
(5) Stop engine and observe vacuum gauge.
(6) If vacuum drops more than one inch HG (33
millibars) within 15 seconds, booster diaphragm or
check valve is faulty.
POWER BOOSTER CHECK VALVE TEST
(1) Disconnect vacuum hose from check valve.
(2) Remove check valve and valve seal from
booster.
(3) Use a hand operated vacuum pump for test.
(4) Apply 15-20 inches vacuum at large end of
check valve (Fig. 8).
Fig. 7 Typical Booster Vacuum Test Connections
1 – TEE FITTING
2 – SHORT CONNECTING HOSE
3 – CHECK VALVE
4 – CHECK VALVE HOSE
5 – CLAMP TOOL
6 – INTAKE MANIFOLD
7 – VACUUM GAUGE
(5) Vacuum should hold steady. If gauge on pump
indicates vacuum loss, check valve is faulty and
should be replaced.
Fig. 8 Vacuum Check Valve And Seal
1 – BOOSTER CHECK VALVE
2 – APPLY TEST VACUUM HERE
3 – VALVE SEAL
XJ
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Troubleshooting
