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I'm having a problem with my brake lights coming on after I park my 51 200. After a couple of hours the brake light comes on and ends up killing my battery. I have to disconnect the battery to save it. Any thoughts?

My first guess would be a defective switch. Another possibility is something in the pedal arrangement -- maybe free play adjustment or spring tension has changed or debris is preventing full return travel -- and is keeping the piston away or allowing it to move away from its normal resting place.

There is a tiny relief port in the master which is supposed to be uncovered when pedal is completely relaxed. If something is keeping the piston away from its end rest spot by even a small amount it keeps that port covered. The fluid being hot will want to expand slightly. With port covered and no way for fluid to expand back into the reservoir, it may exert enough pressure to activate the switch.

Because it happens after a couple of hours and when the car has cooled, the lights don't go back off, I would suspect the switch as more likely. Easy and inexpensive enough to replace as a first step. If that doesn't solve the problem, at least you will have a spare switch.

My 56 had a similar problem that would turn on the brake lights unpredictably after the car was shut down. But it was always a matter of a few minutes, not hours. Replacing the three prong sender switch (located almost under the battery tray) with a NOS switch seems to have solved the problem. But strangely, that cure took a few days to make things work as they should. At first, the old problem persisted, even with the new switch. My prayers might have caused the solution to take place eventually. One of the mysteries of having a 58 year old car.

If you are using silicon brake fluid it may ruin your old switch in short order.

Assuming the switch is good, either pedal free play is absent (adjust to about 1 inch) or the vent hole in the cap is clogged.

Why not try some diagnostics?

Wait until the lights come on and open one of the bleeders. If the lights go out the problem may be due to a sticky wheel cylinder moving and creating pressure that is not sufficient to open the residual check valve.

If they don't go out then consider a problem inside the switch.

If the brakes are adjusted too tight a cooling drum may also create pressure.