I have a 1986 911 Targa that I do alot of DEs and have club raced it. I recently flushed out the brake fluid with ATE Super Blue. I've bleed the system 4-5 times and can't get a good solid pedal. There seems to be a lot of slack in the pedal before I get any resistance (drops below the level of the gas pedal). Once I do get resistance It is not the rock solid pedal I'm use to. The pedal hardness also seems to very from one application to another. Sometimes it also feels like it looses some stiffness while I am pressing on it. I don't have any hydralic leaks. Is this a sign of a bad master cylinder?

 

Sounds like your master may be on it's way out.