I had the same problem with my 88GT that you describe a couple of weeks ago. I determined that it was my clutch master unit. The fluid was bypassing the piston seal and allowing the pressure to bleed down creeping the car forward unless I pumped it up. So, I found the prices for a rebuild kit were around $50 and a new one was only about 20 bucks more. So, if you figure in the rebuild time and making sure your cylendar is clean and a rust free surface, I figured it was faster just to put a new one in. The brand I got was Raybestos. Replaced it and works fine now. Got my new one off of Amazon.com of all places. Rockauto.com has a good assortment including Raybestos.
I bled the slave with a hand vacuum brake bleeder pump and it was fast. Make sure you use teflon tape on your bleeder threads so air doesn't get past the threads for vacuum bleeding.
[This message has been edited by IXSLR8 (edited 11-07-2009).]