I have an 84 300SD that the previous owner did a bunch of work on, including some changes to the vacuum system. The car shifts wonderfully, but when downshifting into 2nd (or maybe it's 1st) there is a single "clunk".

Here is what he says he did:

Engine -

- cleaned and painted valve cover, installed new valve cover gasket
- camshaft valve adjustment
- installed various SS hardware on different engine components
- cleaned up turbocharger, installed new turbocharger gaskets
- installed new K&N air filter on turbocharger
- removed EGR vacuum lines and EGR solenoids from vacuum system
- installed new glow plugs
- cleaned up fuel injector fuel line routing
- tested VCV (vacuum control valve) and leak checked vacuum system lines, checked vacuum system at various points to make sure vacuum in/hg is set to proper specifications
- bled air from fuel system
- recharged air conditioning system using R134 freon
- fabricated and installed custom crank case vent system
- lubricated throttle linkage system
- new filters installed - oil, power steering, fuel

Transmission -

- installed new VCV, adjusted base hydraulic pressure to 42psi
- installed new (upgraded version) B2 piston
- adjusted bowden cable
- changed transmission fluid and installed new filter
- installed new fluid pan gasket

I read that the 300SD uses vacuum on the downshift only, and I suspect something he did is causing the clunk. I replaced the guide rod bushings as they were a wreck and thought that could be where the clunk was coming from, but it didn't help.

Any advice, anyone?