I vacuum pump check valve went out today on my 1983 300SD.
I was able to drive home, without any vacuum for the brake booster, AC or engine shut-off. I can get a new check valve for $24. When I disconnected the line from the pump and removed the check valve fitting, I could see a small spring in the opening where the check valve threads into the pump. I attached a small nylon net to the end of my shop vac, and sucked into the pump. I retrieved the small spring and a small metal piece that looks like a valve. I then started the engine and placed my finger over the line opening, it seems to pull a strong vacuum. Should I just replace the check valve, or should I be concerned that maybe there is still something inside and pull the complete pump assembly? It doesn’t look too easy to get to, and I am not sire that I could get to all of the bolts. My local shop quoted me $400 for a new pump, installed. I guess if the check valve doesn’t fix it, I will have them do the whole pump.
Robert

I recently replaced the vacuum diaphragm on my 79 300td. I didn't know there was a check valve. anyway, I did the rubber diaphragm without following the procedure in the book, and it works great now. I'm thinking that if you feel a strong vacuum, another cause could be a leak elsewhere in the vacuum system, such as a hose, even the door lock system . I found oil going into the air cleaner , and sure enough , it was a ruptured vacuum rubber diaphragm. It sucked enough to get oil out of the engine and into the vacuum lines. Lorenzo..........

At first I thought that there was a leak somewere in the system, so I isolated everything off of the large line directly from the pump. All of the branch lines will hold, but the line directly to the pump will not. I am replacing the check valve today, and we will see.

Tonight I installed the check valve and reconnected the piping.
The pump is producing a vacuum, but only about 10 in. hg vac.
I could not shut-off engine, and A/C only blew through defrost.
I once again checked the main line, still no leaks, but I think that the vacumm should be stronger. Does any know what pressure a solid pump will pull? I guess I will take ot to the local shop tommorow, and hand over some cash.


Robert

I'm going to guess your '83 300SD has the same vacuum pump as my '82 300D. I also experienced a sudden loss of vacuum that killed the brake boost while driving. Fortunately I was near home.

In my case I took off just the front cover of the vacuum pump. Inside the cover you can see a piston that goes back and forth, and three check valves. These interior check valves are about the size of a large button-type hearing aid battery. Two at the bottom are for pump exhaust, and one at the top is to hold system vacuum.

Behind the top interior check valve, I found the spring you mention and a plastic piece that I presume is the check valve. It is exactly the right size such that if it gets caught against the button interior check valve it will tightly block it. It is approximately 1/4 inch in diameter and about 1/2 inch long. One end is round and the other tapered, probably to hold the spring.

These loose parts were discarded, the interior check valves were removed and cleaned with solvant. Put the whole thing back together and have had plenty of vacuum ever since.

That additional plastic check valve with a coil spring is nearly worthless in my opinion, and is just an accident waiting to happen.

Ken300D

I took the car to my local MB shop, and they said the culprit was a leaking tans. modulator diaphragm, and leaking engine shut-off diaphragm. I have not yet heard if the pump is bad, I am holding my breath! Meanwhile, check my other post tonight regarding my 300D's flex disk!
Robert