'81 300SD new tranny goes clunk

[tyl604]

Just put a used tranny in the 81 300SD. It shifts reasonably well and shift points are right so I think I have no problem with the Bowden cable or the new modulator. However I get a bad clunk at every downshift.

I have read all the many posts on clunk and guess it must be a vac problem but I have tested with the mityvac and am not sure what is supposed to leak and what is not.

Breakdown - starting with the rubber Y connection coming from the top of the VCV. The line going to the tranny holds 25psi; the short 2" line from the Y going back to the VCV holds 9psi; so no leak there. Modulator is not leaking. Not sure why the VCV holds only 9psi but the old tranny shifted fine before I ran it out of fluid so I think the VCV is OK.

The line going to the EGR black box does not hold vac (it is now plugged with a golf tee as seen in the picture). The line going to the very large brake booster(?) line holds no vac. Since my brakes work fine, I assume this is designed with no vaccum. The EGR line is now out of the equation.

Summary for top rubber Y - vac at tranny line and vac coming from VCV. The other lines have no vac and I assume they are designed that way.

Now for the bottom rubber Y - no vac. One line just vents to the cabin so must be designed for no vac. The other line goes to EGR black box and has no vac (it is now plugged with a white golf tee too). This EGR line is also out of the equation.

Summary - guess the bottom Y is not designed to hold vaccum.

So does anyone think I have a vac leak in a spot not designed to leak? If not, then why the clunk at downshift?

Appreciate your thoughts. See picture and white golf tees.