The brake pedal on my 1986 F-250 goes to the floor and the "brake" light comes on. The fluid is full. This was not a gradual problem, it happened over night. What could it be? Thanks

The truck has 2 seperate braking circuits, one for the rear brakes and one for the front.

The brake light comes on when there is a substantial difference in pressure between the two circuits. The most common reason for this is that one brake circuit has sprung a leak. You (or a mechanic) needs to trace all the brake lines from the master cylinder to the wheels to find the leak and repair it.

In my experience on older vehicles it is due to rusty steel lines or completely rotted-out flexible lines.

either your seals in the wheelcylinders in the rear wheels have started to leak opr the seals in the mastercylinder have worn out.look at the inside of the rear wheels if you see brake fliud then its the wheel cylinder. don't just replace the side that is leaking .do both sides at the same time. could be a rusted brake line as well. get someone to pump the brakes as someone looks for a leak..how old is the mastercylinder?