I am trying to figure out why MB has two check valves in close proximity to each other(about an inch apart) in the Pierberg vacuum pumps. The first is in the head of the pump and the second is in the brass screw-in fitting that connects the pump to the metal brake booster line.

When I recently replaced my vacuum pump(out of an abundance of caution--original pump in '83 300TD with 210,000 miles) I found the check valve in the brass fitting had broken up and I was able to locate only the black plastic piece and the broken metal "spider" that was supposed to hold things together. So the pump does seem to work without the second check valve.

I tested the new check valve in the new brass fitting with a Mityvac and it seemed to leak quite a bit in the direction that it is supposed to prevent flow--a bit of oil on it improved it, but it still leaked back.

It is tempting to use the brass fitting from the old pump, without any check valve, to prevent damage if the new one fails.

Any ideas why they spent the money to put in the second check valve? They are really expensive to replace if all you buy is the brass fitting.

Peter

I would suspect that the intent was to prevent residual vacuum (when the engine is shut down) from pulling oily crankcase fumes back upstream in the vacuum system.

When you realize that the fitting is actually cadmium-plated steel instead of brass, you will probably be even more shocked about the price.

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I couldn't find repair parts for a price that made trying to fix it worth taking a chance. Valves were leaking. I kept the old pump in case it is useful in the future.

Piersburg is the OE manufacturer and their pumps used to be available for less than OE. Only PE pumps were available when I bought last.

Thanks for the explanation re the second check valve preventing residual oily stuff from getting into the vacuum line. Makes sense. I just wish they had a more robust, effective and affordable design. The plastic check valve guarding the brake booster seals very tight and PP has the genuine MB vacuum hose assembly including the check valve for $28. And you are correct that it is cad plated steel--or some new form of magnetic brass. At almost $100, it may be the most expensive part on the MB on a $/lb basis.

Peter

Usually $75 for used ones on ebay. That is why the outlet check valve is usually missing in the junkyard. It is an easy pick, and I have at least one spare. It is steel, not brass. I am sure somebody could adapt a universal check valve if these ever become unobtanium. Indeed, you can replace the whole vacuum pump with an electric pump from a recent VW/Audi car.

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