I found that it may be too much trouble to fit the motors inside the existing bench with the homebuilt checkvalves. It's using the box idea that Jake and others are using, only with a 3" aluminum door ( round) instead of flapper valves. I just hope I won't be loosing too much from the piping from the external box. But the box will allow me more motors in the future.
I tested what one surplus center motor did in addition to the 7 shop vac motors and was able to get 29" through a 350 plate and way in the 30's with a 285 plate. So the 8 surplus plus 7 shop vacs looks promising, unless the power from the 8 override the 7.
Started on the "secondary box" for extra types of wet testing and using dato'ed joints. Boy that sucker is strong. 1/4 deep dato to accept the 3/4 wood gives it 3 surfaces to seal against. Liquid nails and 2" screws finish off the joint, then a bead of silicone at the corner. I severely doubt any leakage at all even at higher depressions.
Chris.