First things first, do you have on XJ, or a full size like you claim on your info (and btw., the 4.0 is not a V6..)? Second, ABS or standard brakes?
If you have standard brakes and the pedal goes all the way to to floor, you most likely have a bad master cylinder. Booster may hiss a little during braking, but if it's bad you'd have a very hard brake pedal. Air in the system and you've got a spongy feel on the pedal, bad MC makes the pedal go slowly all the way to the floor (and you have reduced or no braking power).
1998 Grand Cherokee 5.9 LX daily driver, 1.75" BB, 32" KM2s, HPD30 Eaton e-locker/D44a stock LSD, 4.56 gears, custom- fabbed tube bumpers and tube fenders,...
1990 XJ Limited (4-door), 4.0 I6, AW4, NP242, ***rolled and totalled @ 165k miles***
1990 XJ (4-door), 4.0 I6, AW4, NP242, PBR 42" tires, Unimog 404 portal axles, 110" WB, full cage + uniframe completely rebuilt, front 3-link + panhard / double triangulated 4-link rear,... ***SOLD***