Brake trouble

[BluTrukker]

I have a 68 non power drums all around, and I seem to have lost brakes. The PO rebuilt the entire brake system except the master cylinder. Brakes have worked great up to now. The other day I went for a short trip and steadily lost braking as the trip progressed. I stopped at a gas station and checked the MC and it was nearly empty in the rear half. I have not torn anything apart as I don't have time for the next few weeks, and I don't see any obviuos signs of leakage. My initial thought is rear cylinders, but there is no fluid leaking out of the drums. Could the master cylinder be crapping out? Anyone have this issue and can point me in the right direction so I don't spend too much time chasing it down? I still have some brakes when I push it too the floor, but not much.