Someone correct me if I'm wrong, but I don't believe the 50's vehicles originally had a vacuum pump. Vacuum was manifold only - and deficiencies are when you mash down on the gas the wipers stop working!
If you have a double action vacuum/fuel pump, you would not use the manifold as a source of vacuum. It's either/or.
I've heard a vacuum canister is a nice addition to this setup, to give a more consistent vacuum at low RPMs. Wish I could advise you where to find one...
-Boyo
