Ford Brakes/Suspension :: Brakes Need Pumping To Build Pressure?

I'm trying to diagnose thoroughly my braking problems before I start throwing money and time at replacing good parts needlessly. My braking system is on a 1971 f-250 4x4 with a swapped in front and rear 1979 dana 60's. Disc front, drum rears. Brake lines are in excellent shape, with no visible leaks along lines or at fittings. The individual rubber flex hoses at each corners are also sound with no leaks or bulging. Rear cylinders and front brakes are also dry. Master cylinder reservoir leaks a little out the seal under the lid, no big deal I have just kept an eye on it. Topped up front bowl from half the other day. Front pads and rear shoes are fine upon inspection. Brakes are in excellent working order.....when they work!*

So here are the symptoms: When driving, if I step on the brakes, the truck coasts to a stop and pedal goes to floor. However if I pump the brakes 3-5 times, pressure builds up and truck stops great. Than after driving again for even a few seconds that pressure is lost. When truck is parked and off, if I pump the brakes, the system builds pressure and the pedal is firm, when I start it up, the pedal pushes to the floor, turn it off and pedal is firm again.

I mean, I can force it down but I refrain. Are my rear brakes adjusted you ask? That was my first thought since for the past month my pedal travel had been increasing. I adjusted them when I was done inspecting them. I put drums on first, than adjusted both left and right simultaneously(posi rear) until drums would not move, than backed them off a little till I could rotate them with pressure. I have yet to open the system up yet or bleed it. Air in the system seems to be the old go to, but I want to be sure that is the problem as these symptoms exist, before I risk potentially screwing something else up.

Even in my research I have gone down a hundred rabbit holes and found all kinds of people who have done the same. Could this be the master cylinder? Another popular culprit. Could it have an internal leak? I have eliminated the brake booster as the problem because it is dry and when I pull the vacuum hose the brake pedal gets hard. I have an aftermarket ssbc combination proportioning valve on the inside of my frame rail, Could the problem be here? Could the fact that it is extreme cold here lately affect that? Could water and road salt get in it and freeze. Last week I went through some ice into deep water, and the brake travel worsened a couple days after that.