Be aware that a master cylinder can fail with the symptoms reported and still not leak any fluid externally.Sounds like a bad master cylinder. My 91 spiders is doing the same thing and its leaking. Replacement should be here today to go on tonight. There should be a small hole in the bottom of the master cylinder at the back about a < inch from the mounting flange, if its leaking you will see fluid coming out. Have some pump the brakes as you look.