How much vaccum is needed for my wipers to work.

[lmdangerous]

I have a 1951 JS code 1 ton pickup. I am in the process of cleaning the wiper motor up and getting as much as i possibly can out of the factory wipers. The motor is a 235 (1957) and it has some wear, but it holds 18 lbs of steady vaccum, other than when you stab the pedal. I know the electric set up is the way to go, but I would rather stay with the vaccum wipers. That would be one less thing to worry about on the electrical system .
LMD