Sounds like air in the system; I'd likely re-bleed the system. However,I would still check very closely for leaks at the calipers and hoses. Check also for a vacuum leak at the brake booster check valve. If you are sitting at a light with your foot on the brake, does the pedal go progressively lower the longer you sit? If so, I would strongly consider the master cylinder. Alternatively, the master cylider is only about $135 at FCP Groton, thus if you are going to open the system to re-bleed (and are worried that the master cylinder is weak), it would be a great time to just change it out anyway. Hope this helps, Jim