Your signature says "1998 shortbed." I will assume that it is a 1989 MJ.
Also, you have an AX15. Is it an internal slave cylinder? I will also assume it is since '89 first got the AX15 internal slaves.
The through-out bearing seal may have a diagonal cut in it. When you drive forward, the cut is closed; you leak fluid slowly.
When you go into reverse, you open the cut; you lose fluid quickly.
Either way, the slave cyl needs replacing.