Testing a Proportioning Valve?

With a booster, definately shouldn't require a lot of foot pressure.
Go to the Master Tech Series 1967-70 and read up on the brake system. It will make the trouble shooting much easier. There's even one on 69 up drums which is a big help these days since so few people really understand what it takes to make them work well.
Master Technician Service Conference - Chrysler's Training for Mechanics

For the booster, AFAIK the shop manual will be the only factory info. If you don't have one, Bishko is a retailer of reprints and sometimes originals.
free digital downloads are available at mymopar.com