Here is an explanation of how the double pump works and what you should expect. 54 Truck Manual It appears that you have more than enough vacuum to have your unit run correctly. I wonder if you have too much friction, as you suspect.

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!