300M, Concorde, LHS, New Yorker Discuss the Chrysler LH platform cars within.

300m -> is there a way to test vacuum brake booster ?

Old 07-28-2014, 08:28 AM
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Join Date: Dec 2011
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Default 300m -> is there a way to test vacuum brake booster ?

I have a feeling that there might be something wrong with my vacuum brake booster. I rarely ever have to make "emergency" stops or slow-downs in my '00 300m, but when I do I seem to have to apply an inordinately large amount of pedal force to slow down the car quickly. Ordinary driving and braking for stop signs and red lights don't seem to require the same force.

I have a '01 Dodge Ram, and even after it's parked for a few days and I let it coast out of the garage in neutral (before starting it) it retains enough vacuum in the booster for 1 or 2 vacuum-assisted brake depressions before a LOT of pedal force is needed.

But for the 300m, after overnight parking (about 8 hours) there is no such vacuum reserve. Should there be?

So I'm going to try the following (from the LH service manual) but I'm not sure what they're saying in step 3. The booster is somewhat hard to see way back under the windshield, and from what I can tell the vaccum check valve has only 1 port on it - with a big hose going straight to the intake manifold with no other ports or connections coming off it.

The cruise control seems to have it's own very small line coming from a separate port on the intake manifold. So I'm not sure where they're indicating to connect a vacuum gauge.

One final thing: I know that there is some sort of test of the ABS/traction control system that is done once each drive-cycle - the first time the car reaches 20 - 25 km/hr I hear a distinct metallic click coming from somewhere, probably a valve or solenoid. Have been hearing this for about a year now. Is there some part that can be replaced that will restore the silence when this test is performed?


(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 temperature, 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 reference 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
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