alright im backing out of my driveway hit my brakes as i come to the end of my driveway and my brake pedal goes straight to the floor. driving down the street and it does it again. then it starts to feel better. then back to the floor. anyway i replaced the mc and it feels a little better but still i get the same problem.anyone got any suggestions?

did you bleed the system and did you bench bleed the MC before installation

If you answer no to Pat'* questions, you should bleed them to get any air out of the lines. That could be why your pedal feels better some of the time.

Also, did you check for leaks before you changed the MC? Did you look inside the rear drums for leaks?

ya.. it a no brianer... mancanic or no mancanic knows when replacing the mc to bleed both the mc and all the lines... un less you have not read the book that came with the mc.

Yes, however it appears that either it needs to be bled more, or something is leaking.

At this point, no brainer goes back and double checks everything and redoes the bleed.

I've seen a faulty antilock brake valve do this too. fwiw.
If you don't find a leak, don't forget to check the brake booster to see if leaking into it, and the line are all bled well then suspect the abs valve.

if you replaced the MC and its still doing it i would say power booster... but im not a mechanic so.... i just try to act like one on the weekend..