91 300D Vacuum Leak (?) Rough Shifting/Banging in Transmission - Valve question

[Rhauss]

Hi,

This is my first post, and I appreciate any help and guidance offered!

I have a high mileage 1991 300D. Currently at 340,000! She's as reliable as can be. About a year ago I had a local shop replace the blower fan for the heat, as it had stopped working. When I picked up the 300 D it was shifting hard. As I'd had the transmission replaced in it's recent history - I returned the car to the tranny shop who said " I think you have a vacuum leak ". I have driven the car and almost gotten used to it. Now as I prepare to move this car on to someone else, I'm trying to clear this problem. Today I returned the car to the shop whom installed the blower fan and they said the transmission vacuum line was in tact and had vacuum. They then pointed to a inline valve (located on the right side of the engine behind the engine shut off switch) that is in the transmission vacuum line and said ... "We think this value should only allow air flow in one direction, but yours allows air flow in both directions" and they speculated that could be the problem.

Does anyone know if that valve in question should be a one way, or in fact should allow air flow in both directions?

again, thanks in advance for helping me with my first post!