I rebuilt my vacuum pump and it is now pulling very little vacuum. Before the rebuild the vacuum gauge was reading around 21. There appeared to be a little oil in some of the vacuum lines so I pulled the pump and the valves were covered in oil. I installed a new diaphragm and I think I put the valves in the correct way as it is now pulling about 3 on the vacuum gauge. Any ideas as to what I might have done wrong. If I pull the vacuum pump again what should I be looking for as to where the problem might be?

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1976 300D 70K

The big external check valve is shot and/or internal check valves installed backward, or air is leaking around the gasket.

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2009 E320 Bluetec 117,000
1995 E300D 306,000 Sold
1996 Ford Taurus LX 130,000 Sold
1985 300TD Still 225,000 Sold
2016 Ford Fusion 24,900

I pulled the main vacuum line and there is little vacuum at the pump. It is sucking and blowing at the correct ports. Could the valves be in backwards and yet it pulls vacuum at the correct port? Is there a way to check for a leaky gasket?

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1976 300D 70K

I think I remember a post about a guy that got aftermarket check valves in his kit too thin to seal. Do not know the type of pump he had or the resolution of his problem I would go to the archives and check it out for the little time investment it takes. Frustrating experience to have.

That is very likely to be the problem. Some repair kits have check valve seals that are way too thin.