Funny brakes. Can't figure it out.

1990 , 4wd, renix, aw4. Earlier this year I tackled the wj master and booster swap. from the beginning the pedal always felt really stiff, hard to press and not real good stopping power. I thought I had it diagnosed to rear wheel cylinders. I replaced them along with shoes, spring kit and complete fluid flush. Before I did the swap I replaced front calipers, pads and rotors. Plus all lines are new. So in essence must everything has been done.

The problem: when I first start it up after sitting for a few hours the brakes work great, softer pedal with lots of bite. Can easily lock up 33s if I need to. After 4 or 5 times stopping the pedal stiffens up. The brakes still work good but not as good. Feels like it takes more push on the pedal and won't lock up the Tires.

Any ideas? I was thinking probably brake booster but all the trouble shooting I have done it tests good.

My first thought would be that the breaks are overheating.

Second would be that the push rod from the pedal to MC is too long and keep a constant pressure on the calipers/rotors causing them to overheat.

More information on the how and when will be helpfull.

Also, when the breaks do not function properly, do a simple heat check by touching the wheels. They should be slightly warm but not hot.

I made a spacer to space the booster out 1/4" so there is no pressure on the rod. I can grab the pedal and give it a slight wiggle.

I would say over heating could be possible but this happens even if I drive 5 ft. example. last time I was camping I started it in the morning drove a couple feet brakes felt great. Then drove to staging area doing 5 mph for maybe 100 ft. again brakes felt great. Sat there with it running for 10 min or so took off drove a few feet and the pedal was much stiffer requiring quite a bit more pressure..

A hard pedal with no assist sounds like a bad booster to me. A simple test is to disconnect the vacuum hose from the manifold (plug off the nipple) and see if it the pedal feels the same way or if its different. Should be very firm with no assist. Don't drive too far.

The fact that it works as first is the only curious symptom. Maybe the check valve is failing, or the diaphragms in the booster are failing, and only acting up after they get warm. Another test would be to pull the plastic check valve out of the booster and listen for a whoosh of air. Do it when the brakes are working and when they aren't working, to see if there is a difference.

Check valve for the fail! Wasn't enough to cause a big vacuum leak but enough to were it want holding pressure. Took it out found a big crack, did a suck test and got all giddy when it was bad lol. Replaced with one I found laying around and Walla good brakes...

Thanks everyone for the ideas..