On a 1989 740T auto w/ Garrett W/C turbo:

I am tracking down the source of the dash heat A/C vent flaps still closing under boost. I replaced the B&W check valve, and that sort of fixed it- that is they don't close so quickly, but they still shut under boost. I think it may be the red and white vacuum check valve.....

I have one on the vacuum line between the Compressor bypass valve (CBV), mounted on the front of the engine block, and the intake manifold. It's oriented with the red end towards the manifold.

From the 700 Repair FAQ:
"If you are having this problem pull the check valve (little red valve that sits in the middle of a vacuum hose from the intake manifold to the CBV on top of the engine) and blow through it both ways. You should get major SOLID resistance (but not full stoppage) going both directions. If you feel a sudden "thwap" as you blow that means the diaphragm inside is probably shot. This applies only to Garrett turbos." [which I have]

Mine R&W check valve does not have "solid resistance" I can blow through it both ways equally, with a resistance that seems to be on par for a small unit like that.

Ideas?