Sounds like you may actually have a bad slave cyl. You say it was all mucky (I guess it was leaking??) and it has a little movement? It may be bypassing under load and only working when you have it off and not under load. Just a thought.
[CENTER][COLOR=White]97 Sport [/COLOR] 4.0l Auto D30/D35
RE 3.5 SF, 1" BL, 1" MML, 285/75/16 (33") MTRs, 16x8 MT Classic Locks, 4.10 gears, Lockrite Front, Warn Front and Rear Bumper w/ Tire Carrier, Warn Diff Skids, Rock-it Man Rockers, Jeeperman Front Hoop, M8000, A to Z winch mount, Hella 500s, H4 Conversion
[COLOR="Red"]Former 3.07 Gears Club Member #1[/CENTER][/COLOR]