Hello, I'm Jeremiah. A car idiot.
I've got a '79 Crysler car with mushy brakes. I'm able to push the brake pedal completely to the floor and the brake light comes on often, sometimes staying on. We've replaced the master cylinder and front brake pads. We've also bled the brakes numerous times. The problem remains.
I can pump the brakes quickly and create presure, but if I wait about two seconds, the presure is gone. The front brakes are disc, the rear are drum. I've not replaced the rear brake pads.
Did you bleed the Master Cylinder separately from the whole system? There could be air in the master or a defective one. Are the hoses and lines all in good shape? If not replace them. Check the rear slave cylinders to see if they are seeping fluid. Obviously, if they are leaking replace them and re-bleed the rear system. Then all you need to do is adjust the rear shoes.
My thoughts are... air trapped in the system.
sounds to me like you might have an internal leak problem in the master
the master cyl contains two circuits usually diagnol(one front wheel and one rear per circuit) therefore it has two seperate pistons and from what you say (losing pressure after you hold the pedal down)sounds like front one is leaking to the rear one.