I experienced a similar failure. The check valve plunger had become very worn allowing the mechanism and check spring to shift out of position.

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Blair Tyson
1985 300D
300,000 + miles

Here is a new check valve as installed on the replacement vacuum pump. Also included are before and after vacuum pressure levels.

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Blair Tyson
1985 300D
300,000 + miles

i installed a new check valve today. i fished around inside the pump outlet but COULD NOT find any pieces of the destroyed valve. i found this very odd, but installed the new valve anyway. it still is pulling way too little when cold-- only 5". the valve however, did seem to make a difference when throttling up.. the vac when throttling up when cold will go up to 15", and the new valve seems to hold it there better than when there was no valve. but at this rate, i need a new vac pump

Saturday afternoon DIY repair of vacuum pump check valve. If you can recover the plunger, spring, and three-eared plate, this may be a short or long term solution for getting your vehicle back on the road quickly. It cost me 17 cents vs. 50 dollars for a new valve.

Got a .406 c-clip from the hardware store (pictured). My local Ace had them. Drilled out the center hole in the three-eared plate to a larger diameter (be careful, as the ears are sharp. I held mine with vise grips while drilling). I stretched out the spring out ever so slightly to make it longer, put the plunger and spring into the valve body, then rested the three eared clip on outer rim edge (it is originally welded to the inner edge) such that it was centered enough that it wouldn’t fall through. A small screwdriver works good for moving the plate around to the right spot. I then inserted the c-clip to pin the plate firmly to the outer edge of the rim. Added a dash of super glue on the plate ears and c-clip for added security (likely not necessary). When the glue dries, you’ll want to confirm smooth operation of the plunger by gently cycling it with a pencil or screwdriver.

You drill out the center of the plate because the c-clip effectively makes the passageway slightly smaller (you want to compensate for that). You stretch the spring because the plate has been moved away from its original position ever so slightly (you want to compensate for that, too).

Hope this helps somebody in a pinch.

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Thank you for posting this fix, Benz in the Road!

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Rich

83 300D
98 E320 Wagon