Found these answers on another thread, guy had similar problems with a fresh jeep build.
- booster to master cylinder rod is too long not allowing brake fluid to fill back to into reservoir fast enough. As the fluid heats up in the lines the brakes apply because the fluid cant go back to the reservoir.
- pedal to booster rod is too long
Booster to Master cylinder rod too long? I'm pretty sure I threw out my older booster and MS to compare to the one that came with the MS/booster combo I bought.
Pedal booster rod was installed on the booster when I bought it.