If the booster is like others I have driven with, removing the vacuum feed should give you a normal manual unassisted system. This would be a good test to see if it is the booster or or the rest of the system.

1951 GMC 1 Ton Flatbed -- It is finally on the road and what a great time I have driving it!
1951 1 Ton Completed

My Chevy Master 4 Door is on the Road!