Sounds like the cups in the master cylinder aren't sealing well anymore if you ask me. When you hit the pedal hard the fluid doesn't have time to leak around them yet, so the pedal is firm, but if you push it slowly it is letting the fluid leak around the cups.
-Intense Blue '99 Durango SLT 5.2L FT4X4
-Ivy Green '69 Plymouth Satellite 318
Somewhere in Kenya, a village is missing its idiot.