Testing a Proportioning Valve?

Mattax said

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

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Thanks Mattix, I fell into a rabbit hole watching those old Master Tech videos! They're awesome. But it just left me thinking the same thing I've been thinking for years and that the issue has to be the master cylinder. Even though I have already replaced the master cylinder.

So next on the list is the hard lines up front, only because I have them and it will be super easy with the engine out of the car right now. I think I will also replace the prop valve, since it will be so easy to do. But (I think) the core of my problem is the front brakes don't grab hard enough, so I'm not sure changing the prop valve will do anything. We'll see, I guess.