Discovered a use for the old pump after seeing that my new pump wasn't so much of a bargain after all (let that be a lesson to y'all!). Turns out the pump won't hold vacuum after the engine is stopped -- the check valve leaks. I took the main vacuum line off of the pump and attached my Mityvac; it pumps down but leaks back in about five to ten seconds.

I experimented with the old pump, discovered that the check valve and piston of the pump work together to prevent air from moving back into the vacuum lines. The old pump will hold a vacuum for days -- I have a gauge permanently installed in the main line and have watched it for months.

I plan to let the new pump be for awhile, maybe the parts will wear in and seal better after a few miles. I don't know if swapping out just the check valve will help. In the meantime, there don't seem to be any ill effects from not having vacuum, since the door locks have their own pump.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970