My daughter's car is a 96 Accord EX. A year or so ago the Master Cylinder began to leak so I replaced it. Soon after this, a strange problem began. When the car was being driven slowly sometimes the brake pedal would go to the floor. A couple of pumps and it was good and firm. This usually happened immediately after starting but eventually began happening more frequently. It never happened at speed, only slow driving like in a parking lot.

Everyone I talked with thought I had a bad Master cylinder. So I replaced it. Then after the problem didn't go away I replaced it again. Then after the problem didn't go away I replaced the ABS system.

Let me point out the following:
We have followed all Honda's instructions on the bleeding process.
The MC's where all bench bled properly.
We properly bled the ABS system before and after replacement.
There is NO fluid escaping the system.
All hoses are intact and look good.
Except for the times when the pedal goes to the floor the brakes feel great with a hard pedal and don't leak down.

After talking with yet another brake expert I decided to replace the MC yet again. This time after bleeding, the brakes acted normal for almost 2 days. Then a few times the pedal went to the floor. Then finally the problem occurred at speed and REMAINED.
How do I find the problem? Right now the pedal goes almost all the way to the floor and the brakes barely work.

 

this does sound like a weird one............
the only time that i have seen pedal go down to floor , is for a leak ,, line, hoses,proportioning valve, calipers, wheel cylinders, brake master leak .......

by the way did you chk the proportioning valve ?? if fluid in the reservoir getting low??? if it is there is a leak ,,,,if not,,, i would have to blame the brake master cylinder yet again.......are you using a good quality brake cylinder ?? ever try a Honda brake master ?? pads and calipers ok....sliding pins ok ??

 

I believe the proportioning valve is built in to the ABS. I have never seen any fluid leaking anywhere. I just did an inspection again and it's dry. No evidence of any past leakage.
Could it be the vacuum booster? The reservoir is full

 

vacc booster if bad would make brake pedal feel real hard to push in ,and not fade to floor , so it is NOT the booster ..........what brand of master cylinders have you used?? new or rebuilt ?
you might want to take this to a shop that specializes on Hondas ,,,,,,,,,,but it still sounds like a bad master to me