Nissan Repair: 2001 Xterra Brakes, piston seal, master cylinder

Question
I have just replaced the brake front pads. Since the Xterra has 75,000 miles on it I replace all the brake fluid and bled the lines. The problem now is that with the engine off the brakes feel great, firm with no give. But when I start the engine the pedal will get firm then slowly drop to the floor. I have rebled the system 4-5 time thinking air had somehow gotten into the lines with no improvement. When driving the anti-lock still works, but as soon as the vehicle stops the pedal drops again. Is there something with the anti-lock that needs to be bled in addition to the lines/calipers? Any help will be greatly appreciated.

Answer
Reed,

If you bled the brakes properly and all the air is out of the system and the pedal goes to the floor you need a new master cylinder.  When you bleed brakes on a car that has some miles on it I recommend using a block of wood under the brake pedal to keep from puching the cylinder of the master cylinder into dead space that has crud built up in it.  Most of us press the pedal to the floor when bleeding and we end up schredding the piston seal in teh master cylinder running it on that part of the bore that has not been used.  So, get a master cylinder and rebleed the system and you should be good.  Don't feel bad as this has happend to me more than once.