I have a 2000 civic EX with 212,000 miles. Brakes were done at 200,000 (calipers, pads, rotors, drums, and shoes) my brake pedal started randomly falling to the floor. The tires lock up right where the pedal hits the floor unless i pump the pedal fast. I figured it was the master cylinder so i repleced it with a remanufactured one. I bench bled it and flushed the system. Well same thing happened so i replaced it with a new master cylinder. Bench bled it and flushed the system. Well the same thing is happening. I guessed that the remanufactured cylinder was defective. I'm thinking the new one is as well, but Before i replace the new one with another new one i was wondering if any of you had any other ideas. I'm completely stumped. Please help.