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Old 11-26-2000, 06:39 PM
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Join Date: Oct 2000
Location: Jacksonvill, FL, USA
Posts: 125
This is a 1981 240D. The vaccuum pump is producing about 17.5 #'s/in. The doors lock and unlock, the car shuts off, if it has been run for a while. But, it I am just pulling the car into the garage it will take a while to shut off and often, not always, I do not have power brakes when I start out.... and at other times too, but not consistently.

Is this vaccuum pump, brake servo or something else?

How can I diagnosis the problem?

Thanks again and any help would be greatly appreciated.
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Old 11-26-2000, 07:52 PM
Posts: n/a
My '79 300SD had the same problems. I had a vacuum hose broken in half going to the first door lock (drivers) it cracked from opening and closing the door so often right where the hoses go into the door. It was the last thing I checked. The PO had reversed the vacuum check valve that stores up the residual vacuum for the door locks so the system could not leak and affect the other components. the check valve finally failed and I had an open system and all vacuum systems were non-functioning. Have you ruled out the booster itself?
Originally posted by TXBill:
Tref my 79 240D started out doing the same as yours, brake pedal hard as a rock during short manuevers, slow to turn off etc. Now it won't shut off at all on it's own ever and the brakes are hard most all of the time.
Tomorrow I'm going to get a vaccuum tester because I THINK there is a leak in the passenger side front door lock. I also think there is a leak in the brake vaccuum line from the vaccuum pump because I hear a faint hissing noise from the front of the car while it's running. So, I can't offer any advice at the moment, but rest assured you are not alone with your vaccuum difficulties.


82 300 SD
79 300 SD
79 240 D

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Old 11-27-2000, 05:36 PM
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Join Date: Oct 2000
Location: Jacksonvill, FL, USA
Posts: 125
I appreciate the feedback, but does anyone have any help on diagnostics?
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Old 11-27-2000, 10:16 PM
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Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
I have not replaced many brake boosters for this. I would suspect a slow leak in one of the vacuum lock elements. You have to test them under very low vacuum and they have to hold.

Remember you have 6 lock elements in that car 4 doors, one fuel lock (I think), and one trunk lock.

Donnie Drummonds
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Old 11-27-2000, 11:12 PM
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Join Date: Jun 1999
Location: Los Angeles, Calif, USA
Posts: 521
If you block off all hoses (including the brake booster) and you vacuum pump is still producing 17.5 in, it is low. If my memory is correct, my 300SD gave a reading of at least 22 inches.

After you use the brake few times pulling into the garage, you use up the vaccuum. The pump is not able to supply enough vaccuum for the brake (due to slow idle plus a defective pump or vaccuum leak) and for the engine shut off. A work around for the shutoff problem is to let the engine fast idle for seconds (to build up vaccuum) before shut off. Now you have to test the pump and the hoses to see where is the problem. My bet is on the vaccuum pump. I think your pump is the diaphragm type. The diaphragm can break or the valves go bad.

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