68 c10 brake problems

[Gav2606]

Hi all. Ive got 68 c10 with s10 front discs and camaro rear axle with druns. I got in the truck this week and the brake pedal went to the floor. It was fine when i parked it up last time. Fluid level is good. It had dual master cylinder with a remote booster for the front brakes. Ive changed the master cylinder and no change. If i pump the pedal a few times a get a good pedal but leave it 3 seconds and then press it and goes to the floor again..i pumped them 40 times and the fluid level didnt drop and no visable leaks anywere. Could this be air in the system if the old master cylinder failed or any other ideas? The strange thing is dual system master cylinder i thought i would only loose front or rear not both. The brakes do work just right on the floor..