Brake System Help-Pedal goes to the Floor!

[duwem]

My 1950 3100 1/2 ton Chevy truck is in storage for the winter, BUT I want to see if I cant get some help so I can get parts lined up for the spring.

Truck has a stock single circuit master cylinder (under cab mounted, bone stock) and stock 4 wheel disk brakes.

Fluid level is fine and does not go down.

Brake cylinders were replaced not too long ago (before I bought it but they are still shiney)

master cylidner looks like a repacement as well, its rusty, but does not look old. Most of the brake lines have been replaced (again before I bought it)

It stops FINE if you use the brakes often, but what happens is if I drive for 15 minutes without using them, when I go to stop the pedal goes to the floor, I release it and hit it again and they are nice and firm.

It didnt always do that. I put about 600 miles on the truck last summer (first summer I owned/drove it) and at first they would work all the time, but towards the end of the summer they started to do the to the floor.




Few things I'm possibly thinking:

-Brake shoes wore enough that the wheel cylinders need too much travel to engage the brakes and the pedal goes to the floor to fill them then the second push finishes the job??

-I need to add a 2-3psi residual valve right off the master cylinder. That may fix it, but why now and not when I first started driving it??

-Internal leak in the master cylinder (no way to know, maybe you guys do?)??

-Air in the lines?? (have not had anything apart at any point)




I need to fix it because at some time the second push to stop will be too late.