Dick,
The purpose of the check valve is to allow the door to open quickly under hard acceleration (with a load on the engine) and then close slowly. If you try to get the door to open just by hitting the gas while the car is parked or in neutral, it won't open much. But, if the car is in gear (especially with a 4sp) and you accelerate, the the door opens quite well. If your car has an auto tranny, and you accelerate hard, the tranny downshifts rather than staying in third gear, thus, your vacuum stays high and keeps the door closed. Unless you have somekind of shift kit which keeps an auto tranny in whichever gear is selected, it is always going to downshift into a lower gear under moderate acceleration. But with a 4sp, it will remain in 4th gear at any speed which causes manifold vacuum to drop to nearly zero under hard acceleration. I have also noticed that there is a distinct difference in the amount of vacuum flow between different repo check valves. I drove Andy's 70 454 at Chevellabration 03, and it required very little acceleration to pop open the door.
