Hello, I'm a newby here. I replaced the vacuum pump diaphragm in my 1976 240D about 5000 miles ago. Just yesterday I nearly ran a stop sign when the brake pedal went to the floor. I pulled the pump apart just now and sure enough there's a tear in the diaphragm, right along the line of the large washer on the vacuum side. What would cause the diaphragm to fail so soon? What orientation should the check valves be in? Could I have put them in backwards? There might be a vacuum leak somewhere in the car since the door locks do not work, could that cause the diaphragm to fail so soon?

Help!

Hi there, welcome to the forum! I have a 74 240d and the pump diaphragm should not fail like that. Mine was replaced by my indy since I bought the car with a torn diaphragm. It's been 8 months now and it's running just fine. I don't have the check valves in front of me but the direction of air flow should be all pointing towards the vaccum line leading to the vaccum pump and brake line since it's negative pressure that is being generated. The leak in the door locks shouldn't affect the vacuum pump's ability to work but it would obviously lower the available vacuum since there is a leak in the door thereby causing little or no vacuum to assist the braking.

I'd reccomend putting in a new diaphragm and disconnect the vacuum lines to the doors to stop the obvious leaking. Check the vacuum tank with a vacuum pressure gauge if you can get your hands on one. If there aren't any leaks just hook the reservoir with the check valve to the tank so that you can have some braking abilities until you get your doors fixed. I drove around for about a week with only -3 psi of vacuum with a blown diaphragm and that was interesting in traffic

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1974 240D 3.0L 4(M) - 250K -OM617.910 + 300D FW + Rajay turbo power!
1977 6.9 3(A) - 84k

The only "issues" it would cause would be leakage of oil from the shattered housing. Now, as far as problems go...

It is pretty unusual to see the piece broken in that area. Usually they break in the "bell" portion.

When you replace the pump you should upgrade the roller bearings to the steel caged type.

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A few possiblities -

Sometimes when assembling one of these types of pump, you need to see if the big washers at the diaphragm have a sharp and a rounded edge.
If so, during assembly make sure the sharp edge(s) face away from the diaphragm.

With some pumps, the diaphragm may need to be stretched a bit, by pushing on the lever, before completely tightening down the diaphragm-cover screws.

Your new diaphragm may have been aging on the parts-store shelf way too long.

Happy Motoring, Mark

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DrDKW

ummm how did a 10 year old thread get resurrected?

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Good that it was brought up, since I came here from another recent thread that referenced it. In other words, such fussing is unhelpful.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Save yourself pump problems in the future. Replace the pump with a piston type from a 126. Same block, just one more piston.
I did mine after too many times messing with diaphragm replacement.

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1984 300SD turbo 126
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