Dodge RAM TRUCK 1500 1995 Service Manual page 179

B R SERVICE BRAKE DIAGNOSIS 5 - 7
this, drain off a sample with a clean suction gun.
Then pour the sample into a glass container and ob­
serve fluid action. If the fluid separates into distinct
layers, it is definitely contaminated.
The only'real correction for contamination, is to
disassemble, and flush the entire hydraulic system
and replace all seals.
MASTER CYLINDER/POWER BOOSTER TEST
(1) Start engine and check booster vacuum hose
connections. Hissing noise indicates a vacuum leak.
Correct any leaks 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 (internal leakage).
(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 pedal is hard, or no pedal action is discern­
ible, power booster or vacuum check valve is faulty.
Install known good check valve and repeat steps (2)
through (5).
(c) On diesel models, vacuum pump hose or
pump component may have malfunctioned. Check
pump output with vacuum gauge and repair as
necessary. Refer to service procedures in Power
Booster/Vacuum Pump section.
(6) Rebuild booster vacuum reserve and check
brake action as follows:
(a) Release brake pedal.' Then start engine, in­
crease rpm to 1500, close throttle, and immediately
turn off ignition.
(b) Wait at least 90 seconds and try brake action
again. Booster should provide two or more vacuum
assisted pedal applications. If vacuum assist is not
provided, perform booster and check valve vacuum
tests. Also check vacuum output on diesel models.
POWER BOOSTER CHECK VALVE TEST
(1) Disconnect vacuum hose from check valve.
(2) Remove check valve and valve seal from
booster (Fig. 1).
(3) Hand operated vacuum pump can be used for
test (Fig. 2).
(4) Apply 15-20 inches (50-67 kPa) vacuum at
large end of check valve (Fig. 1).
(5) Vacuum should hold steady. If gauge on pump
indicates any vacuum loss, valve is faulty and must
be replaced. '
BOOSTER
CHECK
Fig. 1 Vacuum Check Valve And Seal
J9005-80
Fig. 2 Typical Hand Operated Vacuum Pump
POWER BOOSTER WACUU1 TEST
(1) Connect a vacuum gauge to the booster check
valve with a short length of hose and a T-fitting (Fig.
3).
(2) Start and run engine at idle speed for one
minute.
(3) Clamp hose shut between vacuum source and
check valve (Fig. 3).
(4) Stop engine and observe vacuum gauge.
(5) If vacuum drops more than one inch vacuum
(33 millibars) within 15 seconds, either booster dia­
phragm or check valve are faulty.
TESTING DIESEL ENGINE VACUUM PUMP OUTPUT
On diesel engine models, a low vacuum condition in
the brake booster will cause the brake warning light
to illuminate.
The following test checks output and condition of
the vacuum pump and interconnecting hoses. How­
ever, a more comprehensive testing procedure is pro­
vided in the Power Brake Booster-Brake Pedal-
Vacuum Pump section. Refer to the procedure for
"Diagnosing A Low Vacuum Condition" in that sec­
tion.
(1) Inspect vacuum pump and booster hoses and
connections. Make sure hoses are in good condition
and securely attached. Run engine and check for vac­
uum leaks. Replace leaking hoses before proceeding.