I recently overhauled the brakes on my B2B ('50 1/2 ton). I was careful to bleed the system completely (or at least I thought I so). I bench bled the master cylinder prior to installation. But I find that when it's hot out and after I've gone several miles, the pressure builds up in the system and the brake lights come on without pushing on the pedal. (That is, they stay on after several pedal applications) Is there a special brake fluid required for these systems? I'm using reg. Dot 3 fluid presently. If not then, is there a fool proof way of bleeding the system to ensure no air remains in the system?
Appreciate any input...

Are just the 'lights' coming on, but the brakes aren't being applied? The truck doesn't begin to slow down and stop on its own, does it? The little switch on the brake pedal, causing the lights to come on, has been the culprit of many a dead battery!! The switch itself can be adjusted so the lights don't come on until the pedal is actually being applied. If it's the actual brake system itself, Dot 5 (expensive) is (I believe, help me out here someone...) silcone based, and has no water content in it to help rust lines and cause other unseen problems. I've had older Jaguars and their manuals always say not to use anything other than Dot 5. Any brake fluid will work, but they want to keep moisture out of the lines.

Best of luck with it!
Dana

The brakes do drag when this happens. I'm starting to think I have air in the line. As it expands, the brakes are partially activated and it's enough pressure to activate the switch.

FYI, Finally got around to dealing with this problem. It turned out to be the pressure relief orifice in the Master Cylinder. It was plugged from years of rust.
Drilled it out and now brakes are fine. Thanks for the imput.

I had the same problem with a new master cylinder some I read the sevice manual again. That hole the other guy mentioned was to far in so I readjusted the push rod on the master cylinder.