Run down. 98 Cherokee 4.0 aw4. It came stock with abs I put axles under out of a 94 so no more abs. I have not had a problem running this setup . Went wheeling today brakes worked great the trip up and back about a 6hr drive total. While wheeling in 4lo first gear I would loose pedal it would go to the floor pump it a few times it would come back. So I stop crawl under the jeep check for broken lines bad wheel cylinder or any bad fittings all looks good check the fluid its still full. Wheel for a little while longer going slow and on flat ground LOL brake are fine for about an hour and it happens again. This time goes to the floor one time when I let off the brake and press it again I have pedal again. Not sure what's going on master cylinder maybe?

 

Give the brake system a full bleed, starting at the master cylinder

 

Thanks not sure that's the problem. The brakes are working great know only seems to happen in 4lo. It's weird if there was air in the system you would think that they would be sponge all the time in this case they are not. The pedal is hard and will lock up the 35s on dry pavement.

 

Classic failing master cylinder.

 

New master cylinder and a really good brake flush has seemed to do the trick. Thanks guys.