Yea.... More than likely it is one of the check valves. There is a big one in the top of the pump that "screws in" from the top to the main vacuum line. Then there are a couple more behind the front cover.

I remember doing a "half as@ed" rebuild on mine when I had it. It is kind of a PITA to remove the front cover and replace the check valves and o-ring. BUT, worth it IMO if you are driving the car a lot. I got the kit off of this site. Call them! Talk to them! They will get you what you need.

You really need a good vacuum tester and gauge for this. Harbor Freight is good enough to fix the problem and get beyond it. Just don't expect the tools to last. They got a knock off Mighty Vac tool that is good enough for a couple years. It falls apart after that.

If you plan to keep the car, get a Mighty Vac (or equivalent). Or, start saving big stacks of cash to take it elsewhere.

If you took this particular car to a "so called mechanic" and "he did not hear any hissing".... You need to find another mechanic ASAP or start saving big stacks of cash....

That is not a mechanic. That is a "want to be mechanic". A real mechanic could have hooked up a vacuum gauge in less than 5 seconds on this car and showed you a reading. Then he could have pulled vacuum from the pump and in other directions to check various check valves in about 50 more seconds.

Seriously.....

__________________
AJ

1985 300D (SOLD)