We checked the lines and the cylinders and all have correct numbers on them and they seem to be working ok

 

If the flexible line(s) were the culprit(s) they would hold pressure at the wheel cylinders and have no affect on the brake lights.

Now you've given us more information when you state that the RR gets hot and things return to normal after it cools down.

If there's enough pressure to keep the lights on and make the pedal hard I'd expect all of the wheels to be hot/applied. That you single out the RR kinda makes me think you may have two problems.

When I say to check the pedal for binding I'm not just concerned with the pedal rubbing something but also that it pivots freely, the spring returns it to fully released and allows the master cylinder piston to fully retract. Same thing happens if the pedal rod to master cylinder is adjusted too tightly - there has to be enough space to allow full retraction of the master cylinder piston when the pedal is released. That brake light switch is right there at the master, not between the hoses and the wheels. If the lights are on there's pressure being applied to the entire system. If the lights are on and the pedal is hard there's a considerable amount of pressure in the system.

So, where is that pressure coming from? Well, a lot of heat will cause the fluid to expand and build pressure. If that heat is coming from the brakes I'd be expecting to read you stating that there's a lot of smoke or smell of burning, but you haven't. Just how close is that exhaust? And even at that, if the master cylinder piston was fully retracting and allowing the fluid to return to the reservoir there wouldn't be any pressure build-up and the brake lights wouldn't be on. Unless the vent in the cap was plugged and the master cylinder was overfull. If that isn't the case then we're back to the brake pedal binding or the pedal rod adjusted too tightly...

 

Makes sense except if I'm not driving the pedal never gets hard to push. I'm guessing the exhaust is within an inch of the brake line. I will check the vent cap but again if I can apply the brakes a hundred times with out the engine running and not have the problem doesn't that void the idea of a plugged vent hole or the pedal/master having a problem? Every thing we try just seems to be OK unless the engine is running. As they are original brake system and no vacuum assist the only thing I can think is causing the problem is heat. Going to move the line tomorrow and see what happens.
I do appreciate the help and ideas, please keep making suggestions one of them has to solve the problem.

 

I would replace that brake cylinder. Just maybe?

 

Sounds like a puzzle alright. I'm not sure of your setup but there is most likely one flexible brake hose to the rear. If so I would clamp that and then try driving about and see what happens. Then you can completely rule out the master cylinder and the pedal assembly if it stops happening.

 

The plan today is to replace the right rear cylinder AND move the brake line further away from the exhaust. I like the idea of isolating the rt rear and testing it, good idea thank you. I will post the final out come as this may happen to some one else along the way and it could make life easier for them.
Thanks, again please keep ideas coming until (and even after) I get this solved.

 

If the right rear is getting hot that is an easy place to start looking. I am assuming you have an F3 Heavy Duty Pickup, correct?

 

As Ross suggested. I'd check the master cylinder push rod adjustment. It sounds like you don't have enough free play, and when the brakes get hot, they are self applying. I usually start off with 1 in free play (in the pedal). Good luck.

 

The brake pedal has about 1 to 1-1/2 inch play before it starts applying the brakes when the engine is cold. It starts getting harder the longer I drive it. I took it out yesterday on a road that I don't have to do a lot of stopping so I am not applying the brakes. After 20-30 minutes of driving the pedal is right at the top and very hard to push and get the brakes to work. That is why I think it is a heat problem from the exhaust but that is just a wild guess.
Correct 'Nice-ol Dave'

 

if the fluid was getting heated and boiling you'd end up with a soggy pedal when it cooled

 

CBeav makes some good points. If there isn't a problem in the right rear I would be very suspicious of the "Rebuilt master and wheel cylinders" you mentioned.
I once had a problem where 1 brake was getting hot. Driving me nuts! Later found out the other 3 weren't working! (Just wanted to help confuse things.) It will be great to hear how you solve this riddle. I am currently restoring a 49 F3 and working on the brake system.

 

So far everything that has been suggested has checked out as being OK.
The master push rod does not seem to be binding.
The pedal travel is about an inch and a half before it gets pressure.
We put it up on a lift with the rear hubs off and pressed the brake multiple times and both sides seem to work & retract fine.
We rehoned (sp?) the left rear cylinder put it back together and it still leaks a little -ordered a new cylinder for it.

The only time the pedal become hard is if we drive it for about a half hour - even driving it on a highway with out applying the brakes. Then we turn it off and the brakes lights are on & the pedal is hard -- let it cool for an hour or so the lights go off and the pedal is normal again. We pumped the pedal about 100 times and it did not get hard. With the brake on we tried to push it and we could not. So we think the brakes work if the engine is not hot.

The only thing we have not tried is moving the brake line to the oppsite side of the axel to get it away from the exhaust. Work has gotten in the way yesterday and today so we did not have time to do that. Now that we know we are definitly replacing the wheel cylinder we don't want to have to bleed the sysmem twice so we will wait for the wheel cylinder to come in and do both things at the same time.

I really appreciate the help you all have provided. Any other ideas are certainly welcome -- at this time we are willing to try anything that might work. If need be I am going to get a new master but hope to not have that expense if it is OK after moving the line. Thanks again.

 

I think this is the key, the brake switch won't illuminate without pressure on it.

 

I agree in theory this should be the problem but why doesn't the pedal get harder to push when the engine is not running - it does not have any vacuum assist - and why would it get harder running it down a highway with out pushing the brakes. We pull it out of the garage and about a mile away with only 2 stop signs between the garage and the highway which we just gear down and coast thru so we haven't touch the brakes since in the garage. The pedal was fine before we left the garage after a half hour of driving the pedal is right at the top and very hard to push. The only thing we can come up with is heat from the exhaust pipe is making the fluid in the line expand. The master is not letting that fluid back into it so the pedal gets hard. Maybe it's Viagra? It defies the logic.
I'm not trying to be a smart a$$ it just this thing has been driving me nuts for two weeks. I'm about to say the heck with it and switch to disk all the way around.

 

You can still have freeplay at the pedal, but no freeplay in the rod.