Had the same thing on another make of car. It was the master brake cylinder. BE CAREFUL. Mine worked again for about 2 weeks after bleeding then acted up again. This happened about 4 times before the total final failure. Your master brake cylinder could be failing. When you drive leave enough room in front to react if it fails in traffic. Be prepared to hit the emergency brake if needed. Mine finally quit forever when I had to hammer the brakes on - they held for a second then the pedal went to the floor - never to work again until the new master cylinder was installed. Forget trying to diagnose while they work - it won't show anything. Good luck