So I thought I had this fixed after bleeding all the fluid out of the brakes.
They have worked fine for a few months and then yesterday the first reasonably hot day of the year I drive to TSC with no problems, I park. I go int the store to get feed, come back out get in start to drive and a brake is dragging.
I am glad I did not put the new rotors on!
I am going to rebuild the calipers which I am pretty sure are not the problem.
I have a new master cylinder I bought awhile back. I am going to bench bleed and replace.
For such a basic brake system it is a PITA!
I mean all there is, is a master cylinder, proportioning valve, lines and calipers.
Front flex lines have been replaced as well as all four calipers/pads. The calipers were lubed up, been bled to death. The only thing left is the master cyl and proportioning valve, and I have never seen a bad proportioning valve and since it has no moving parts it is highly unlikely to cause this issue.
If it were not for these brakes I could probably like my Jeep. LOL

Oh no!

LOL, yeah, I tell ya if this was a decrepit horse I'd put it out to pasture.

I've only seen collapsed lines and worn out calipers cause this. I've been working on the brakes of my scrambler for many weeks (only for a little time on the weekends) but am close to having them working properly...my desire to keep working brakes is very low right now.


Good luck, I hope you find something easy to fix it.

Yep, lines and calipers were my thought a few years ago. They have been replaced with new. I am going to rebuild the calipers this time. I am going to go ahead and replace the MC also. My thought on it, is it may not be releasing pressure. It's just odd it only does it when it starts getting hot outside and then not all the time.

Are you sure the plunger in the master cyl is returning all the way?

I have had the plunger issue with other vehicles. When they they had plunger issues I had an issue everytime I drove, not unless it was very short distances. This Jeep will not do it in cooler temps, only when it is hot outside and then it's very intermittent.

Planning on working on the brakes today. I don't know what I'll get done today. I am going to change the M/C and check the pushrod length sine the M/C will be off. I also want to rebuild the calipers and replace front rotors, maybe rear rotors also if i don't run out of gas. LOL If I do, the Jeep isn't going anywhere. LOL
At least it will be easier to get the wheels off since I have the new hi-flo fittings and good hoses. I can use the impact gun and knock them out quick like an old retired disabled NASCAR mechanic in a wheel chair missing fingers and one leg, blind in one eye, cataracts in the other, having a lack of enthusiasm to work on anything anymore. I suppose I should get started before this is all I do today. LOL

I did not get far with the brakes this morning. I bled these brakes to death this past winter. I insure I did not overfill the reservoir. This morning I went out starting to remove the M/C and I noticed it was overfilled, within approx. 3/8ths of an inch from the top of the cap. I do not know why it had that much fluid, especially when it was at the correct level this winter when I bled. Anyway, I took out around 5-6 cc of fluid.
There is not suppose to be any thermal expansion of the fluid, or very little.
So I stopped, and I will drive and see if this does anything, if the very intermittent issue returns.
Like I said before this issue shows it self more in the heat of summer and it's been getting hot here, especially the time of day I drive.

It may also explain why I can drive somewhere and it's fine, come out start driving and the front brake or brakes are dragging.

One thing I did see is where the proportioning valve is mounted. I never noticed it till today. LOL
Oh Well, It may be another week or two before I drive again.

I suppose I'll go bike riding now! Burn off all this energy an old retired disabled NASCAR mechanic in a wheel chair missing fingers and one leg, blind in one eye, cataracts in the other, having a lack of enthusiasm to work on anything anymore.

Oh, I'm not a NASCAR mechanic, but the rest or certain parts fit. LOL

Check the caliper mounting bracket slide rail for wear too... newer vehicles have stainless wear pads, the TJs did not, and they get a divot that can hang the caliper, needs a bit of weld or a stainless sleeve to allow the caliper to slide