Brake problem question

[Mdavis83]

I just picked up a 72 and it has a brake issue. When you use the brakes at first pressure they are soft and then they just about throw you through the windshield. The truck has sat for the last 2 years before I got it but I believe the previous owner had the same issue before it was parked. It has factory power brakes with front disk rear drum. I know the master cylinder is leaking but I don't think that is the issue. I'm thinking it is possibly an issue with the prop valve or maybe just the slides for the front calipers hanging up and then sliding all of a sudden causing the soft pedal, then hard braking. The truck does have a comp xe 268 cam, but I don't think it is radical enough to cause vacuum issues. Still trying to figure everything out on this truck. Any thoughts or suggestions?