That makes sense but why would they work fine with the engine not hot but not work when it is hot?
The only difference between being normal and being hard to push is the engine has been running for a half hour or more.
It also worked fine on the rack with hubs off but engine was cool.

 

Don't have answers for "If A, why not B" but, there's no getting around the fact that a pressure switch works on pressure, there has to be something causing that between the pedal and the master cylinder.

 

take a real good look at that RR brake line. Try bleeding that cylinder to assure fluid is moving unrestricted or if fluid is even moving through it.
When they redid all the brake lines did they run the lines around the rear spring U bolts or did they run them between the rear axle hsg and the u bolts?
If they did the latter you could have a crimped line.

These brakes overall are a very simple hydraulic concept. It does sound as if the master cyl. rod is adjusted to tight or the bushing in the pedal is worn causing variation in the rod adjustment.

One other thing , the brake pedal has 2 holes, an upper and lower, why I do not know, the master cyl. rod on mine is in the upper hole, it lines the rod up straighter to the cyl, I don't think hooking in the lower hole would cause a bind however ???.

As for heat causing the issue it would have to be from either brake not releasing or the exhaust. The exhaust I seriously doubt.

Keep looking You will get it figured out.

 

Tedster9 wouldn't the fluid in the line heating up expand and put extra pressure on the switch to turn it on? As this fluid would then be expanded the pedal could not push more fluid into the line and therefore be harder to push the hotted it got?

JIM & AUSTIN the brake worked fine when we had it on the rack but not heated up from running. We did check all of the lines and none are crimped. The only thing is the exhaust is very near the RR brake line. Hopefully tomorrow will allow time to see if moving the line helps. Work has just gone nuts these last two days so I don't have time to get back in the garage. If not tomorrow then asap.
This just seems to defy logic of a simple brake system. I will also double check which hole the rod is in. I appreciate the help, thank you

 

Not trying to beat up on you, I'm really not.

But, you should listen to someone who has 45 years of professional brake experience and is also nice enough to offer their advice. I see this a lot - people ask a question and then they want to argue with a genuine expert - not you necessarily - and then look for someone else to provide an answer they can agree with.

He mentioned something to the effect the master cylinder would have to be sealed up really tight to answer your question. Usually there is 3/8" to 1/2" room at the top of the reservoir. Lots of expansion for that to happen.

 

Tedster9 I'm not saying you or anyone is beating up on me or you that your answer could be wrong. I don't know the answer and when some one does give me an idea I want to be sure I understand it completely. My question was just that a question that I do not have the answer for or the experience to answer. The fellow that is helping me with this has been restoring cars and trucks for 45 - 50 years and is completely at a loss why this is happening. I sincerely apologize if I offended anyone. This forum has been a great source of information and great fun for me.

 

After reading all the tests you have run and the results, I am beginning to believe that the master cylinder is at fault. Something (dirt or other contaminate) may be blocking the compensating port within the master. Perhaps the piston is sticking and not returning to the full rear position which would cause the issue and still allow the pedal free play you have. All this assumes that you actually have the free play you think you have. Sometimes with the heavy brake pedal return spring, it is hard to feel when the rod actually contacts the piston. Without that spring on you can pull the pedal up by hand (should be against the pedal stop) and the weight of the pedal (if there is no binding) should bring the rod down in contact with the piston. Measure the pedal travel for your free play. Adjust rod as necessary for your 1" . Re-attach the pedal return spring. Have good freeplay and still have the problem? Replace/rebuild the master.

Good luck!

 

Im new to this thread because of intermittent computer problems, and Im not any kind of brake expert. But after all this frustration you have experienced, could you buy a length of pre fitted brake line at Napa and do a sloppy bypass to get away from your exhaust. It doesn't need to be pretty just a temporary piece to get it away from the exhaust heat. It would be cheap probably $6 for the tube and the cost of fluid to re bleed twice. One to clear the temp tube, and once to re connect the original line after the test. Im thinking that it would at least eliminate or confirm your thoughts about exhaust heat expanding the fluid. This is one of those hair pulling, very frustrating things that once solved makes sense. Don't give up you have a lot of very smart folks with you on this one, not me but the other guys.LOL

 

Doug removing the return spring and checking the pedal and rod is something we have not done. Thank you and we will try it.

 

Jimmy - don't sell yourself short on knowledge. I've followed a number of the threads you have done on your truck and they all have come out very well. We decided moving the line permanently won't hurt anything so we are doing that and have already got the line made up to fit the move. I should have just plugged that line to the RR a week ago just to eliminate the possibility of heat being the problem. Thanks, I do appreciate all input.

 

I had a situation with mine where the brake pedal kept getting higher and harder as I drove. I could not figure it out so I took it to a repair shop. The guy told he would only work on it by starting out with the wheel cylinders, replacing them, adjusting the brakes and bleeding them and see if that works......it did and all is fine that was 5 years ago, I now put new fluid in there every two years using speed bleeders

 

That's exactly what the problem sounds like!! For whatever reason, the piston in the MC isn't uncovering the port when brakes are released.

 

In order for pressure to remain applied in the system and the brake lights to remain on the fluid fill port from the reservoir to the cylinder (in the master) is blocking the fluid return path or the reservoir is overfull and the vent is completely blocked. The things that can prevent the fluid return path to be blocked are a pedal that isn't completely returning to a released position, the adjustment rod is incorrectly adjusted or the piston in the master cylinder isn't fully returning past the fill port. The fill port could be clogged, never allowing fluid in or out of the reservoir, but that would be odd considering the hot fluid is building enough pressure to make the pedal hard yet not enough to dislodge the obstruction.

 

We will try all of these to see if any are the problem. I will post what ever we find Thank you to all that have tried to help.

 

IT'S FIXED !!!!
We rechecked the master cylinder and all looked OK.
We rechecked each the wheel cylinder and all looked OK.
We rechecked each brake line and all were OK.
We took the rear wheel hubs off and tried the brakes once they worked fine.
We started the engine and let it run for 30 - 40 minutes and we watched the right rear brake start to engage with out stepping on the brake !!!
We turned the engine off and let it cool and watched the RR retract.!
We moved the RR brake line from the back of the axle to the front. The exhaust
pipe U part came down the back of the axle so the brake line was with in an inch of the exhaust. Now it is 3-4 inches or more away from it.
We tried the brakes 1 time and they were ok.
We started the engine and let it run for 30-40 minutes. The brakes did not move.!
We put the hubs back on and took it out for a 30 minute drive and all was well.

The only thing we can figure is Heat was expanding the fluid in the right rear line and the longer we ran the engine the hotter it was getting. The brake pedal therefore was getting harder to push as there was no place for the hot fluid to go.

This has been the most frustrating problem I have come up with on this and any other project I have ever done.
I want to THANK everyone who helped with a great bunch of things to try. I apologize if anyone thought I was not grateful for the help by asking questions to be sure I understood what they were suggesting. We tried just about every suggestion except replacing the master and still could not solve the problem. I just wish I was smart enough to realize the line was too close to the exhaust days ago. Again thank you all. I owe everyone that helped a good cold soda pop or other libation of your choice! Have fun

edit - Jolly Roger Joe asked if we replaced the boiled brake fluid and we did when we rebled -we took about a quart out just to be sure all of the old stuff was gone. Thanks Joe.