Chrysler 300M Service Manual page 150

LH
POWER BRAKE BOOSTER (Continued)
The power brake booster can be identified by the
tag attached to the body of the booster assembly (Fig.
73). This tag contains the following information:
• the production part number
• the date it was built
• who manufactured it
NOTE: The power brake booster assembly is not a
repairable part and must be replaced as a complete
unit if it is found to be faulty in any way. The power
booster vacuum check valve is not repairable but
can be replaced as an assembly.
Fig. 73 Power Brake Booster Identification
1 - VACUUM CHECK VALVE
2 - MASTER CYLINDER PUSH ROD
3 - POWER BRAKE BOOSTER ASSEMBLY
4 - MASTER CYLINDER MOUNTING STUDS
5 - PART IDENTIFICATION TAG
OPERATION
The purpose of the power brake booster is to
reduce the amount of force required by the driver
(foot-pedal pressure) to obtain the required hydraulic
pressure in the brake system to stop the vehicle. This
vehicle utilizes a Bosch power brake booster to
accomplish this task in all brake applications.
The power brake booster is vacuum operated. The
vacuum is supplied from the intake manifold on the
engine through the vacuum hose and power brake
booster check valve (Fig. 72).
As the brake pedal is pressed, the power brake boost-
er's input rod moves forward. This opens and closes
valves in the power brake booster, creating a vacuum on
one side of a diaphragm and allowing atmospheric pres-
sure to enter on the other. This difference in pressure
forces the output rod of the power booster out against
BRAKES - BASE BRAKE SYSTEM 5 - 43
the primary piston of the master cylinder. As the pis-
tons in the master cylinder move forward this creates
the hydraulic pressure in the brake system.
DIAGNOSIS AND TESTING - POWER BRAKE
BOOSTER
BASIC TEST
(1) With engine off, depress and release the brake
pedal several times to purge all vacuum from the
power brake booster.
(2) Depress and hold the pedal with light effort (15
to 25 lbs. pressure), then start the engine.
The pedal should fall slightly, then hold. Less effort
should be needed to apply the pedal at this time. If the
pedal fell as indicated, perform the VACUUM LEAK
TEST listed after the BASIC TEST. If the pedal did not
fall, continue on with this BASIC TEST.
(3) Disconnect the vacuum hose on the side of the
vacuum check valve that leads to the speed control,
then connect a vacuum gauge to the open vacuum
port on the valve.
(4) Start the engine.
(5) When the engine is at warm operating temper-
ature, allow it to idle and check the vacuum at the
gauge.
If the vacuum supply is 12 inches Hg (40.5 kPa) or
more, the power brake booster is defective and must
be replaced. If the vacuum supply is below 12 inches,
continue on with this BASIC TEST.
(6) Shut off the engine.
(7) Connect the vacuum gauge to the vacuum ref-
erence port on the engine intake manifold.
(8) Start the engine and observe the vacuum gauge.
If the vacuum is still low, check the engine tune
and repair as necessary. If the vacuum is above 12
inches, the hose or check to the booster has a restric-
tion or leak.
Once an adequate vacuum supply is obtained,
repeat the BASIC TEST.
VACUUM LEAK TEST
(1) Disconnect the vacuum hose on the side of the
power brake booster vacuum check valve that leads
to the speed control, then connect a vacuum gauge to
the open vacuum port on the valve.
(2) Remove the remaining hose on the vacuum
check valve that is not the vacuum supply hose com-
ing from the intake manifold. Cap off the open port
on the check valve.
(3) Start the engine.
(4) Allow the engine to warm up to normal operat-
ing temperature and engine idle.
(5) Using vacuum line pliers, close off the vacuum
supply hose near the booster and observe the vacuum
gauge.