txjeeplover

It's your brake booster or check valve. The master cylinder is attached to a large round drum and that's your booster. The booster is what provides power brakes to the Jeep.

"HARD BRAKE PEDAL Lack of power assist may be due to low engine vacuum, a leaky vacuum hose to the brake booster, or a defective brake booster. The booster is located between the master brake cylinder and firewall in the engine compartment. Sometimes a faulty check valve will allow vacuum to bleed out of the booster causing a hard pedal when the brakes are applied. This condition can be diagnosed by starting the engine (to build vacuum), shutting it off, waiting four or five minutes, then trying the brakes to see if there is power assist. No assist means a new check valve is needed."

Castalone

X2. You are losing vacum in the brake booster. It should hold vacum till the next time you start it. Check for leaks in the booster and hose to it. Also is your steering wheel straight? Your abs will kick in early during a turn if it is off center.

Jae Suite

^^^thats what I was trying to say but typing on my phone is a pain

medic_23387

checked it this morning it takes over 3 hours for the pedal to stiffen up

itasor

My JK did the same exact thing for about a week. I was just getting ready to look at the check valve or bring it in, when it stopped doing it. Hasn't happened again in over a year and a half, so I'm not sure what the actual problem was.

oldtimejeep

Why don't you use the E-brake before you start your jeep. You need the vacuum from the engine for the pedal not to be hard.