iv got the same problem with my pedal. not quite as bad. i asked the same question earlier today and yeah the master cylinder is most likely shot. also, i called autozone to see how much one was and asked what else i should check before spendin some money and they said see if there's any water in the booster. just take off the covering and dump it out.
94 sahara, 4" rough country, 1 1/4" home made shackles all around, 33" trxus m/t, home made cai, hesco tbs, flowmaster super 40, no track or sway bars, 4.0 h/o, home made rocker guards, re-routed the trans vent
GIT R DID, GIT R DONE