Brake problems???

[Kantosensei]

I have bled my brakes and they are tight --temporarily. I don't see any leaks. As long as I pump them they work fine. When I hold the brakes they stay tight for about fifteen to twenty seconds, then slowly, the pedal goes to the floor and the brakes release. Also, the brake light comes on. The brake pads are in pretty good condition, about two years old with less than ten thousand miles. Is there any way to test each component or should I replace everything. I would imagine that all of the parts are original. I have owned the Blazer about fifteen years and I know I have only changed the pads. Any ideas would be appreciated. Thanks David