Any leaky from the clutch MC or SC (which may be hard to see... think that's an internal slave cylender model). Could require replacement of either/both.
I just replaced both on my '95 YJ (4.0, AX-15)... seemed to have a bad internal spring/seals. Was developing lots of slack in the pedal after in-city traffic driving, clutching. Eliminated the problem, replacement firmed up everything.
Now: '95 YJ S / 4.0, our latest project
Priors: '93 XJ UpCountry, '79 CJ-7, '99 WJ Limited (2x), '99 TJ Sport, 88XJ; 91YJ; 88YJ; 81 CherokeeChief