Question for brake gurus...

[Greenlee]

I suddenly have brake problems on my 76' Chevy after pulling a tandem axle trailer and tractor weighing probably 4,000 lbs. Coming up to a red light and getting on the brakes pretty hard, the pedal went almost to the floor and the brake light came on for a second and went off. Before this incident, the brakes were fine. After unloading the trailer I noticed the pedal was still going almost to the floor and the red brake light came on again and stayed lit. I check the brake fluid and the big reservoir was a little low. I filled it with fluid, but the light is still on and the brakes are still working poorly with the pedal going almost to the floor.

The truck is not pulling to either side, and I have checked all four wheels and don't see any obvious signs of leakage. My first thought was that maybe the master cylinder has gone bad, but I am not sure how to test it. Does anyone have any ideas?

Thanks