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2002 honda civic brake problem - replaced mast. cyl. and it's still doing it

Hi. I have a 2002 Honda Civic EX 1.7L. Recently the brake pedal would sometimes go to the floor when I'd apply the brakes. The car would still stop, but not as quickly as normally. If I apply the brake and it goes to the floor I would release the pedal and reapply the brake and it would stop at it's normal position and the brakes felt firm and the car would stop fine.

I was told my problem was the master cylinder so I bought one on Sunday and replaced it. I bled the lines and it seemed like all the air was out. The brakes worked great and the pedal stopped in the same place and was firm. Now today on my way to work the pedal went down a little farther than normal during one stop. On the way home it stopped in different places. The brakes would always grab and the pedal felt fairly firm wherever it stopped. I drove it this evening again and the pedal went to the floor like with the old master cylinder. One pump and they grabbed in the normal spot.

Any ideas of what is going on? Did I not bleed the lines well enough? Is the new master cylinder just junk? I saw in the manual that air can get trapped in the ABS sensor, but an air bubble problem should just make the pedal squishy, not change where it grabs, right? I'm stumped and I really don't want to take my car in if I can fix it.

Couple things I forgot to mention: brake fluid isn’t leaking anywhere that I can see, and the fluid level is staying the same. Also, the brakes are in decent shape. The shoes in back are over 1/8" thick (1/16" is min) and the pads up front are over 3/16" (1/8" is min). Drums and rotors are smooth and seem to be in decent shape too.

Did you bench-bleed the master cylinder? The one’s I’ve gotten include a kit with hoses and plastic adapters to allow you to bench bleed the master before you install it. This is an important step not to be skipped.

Yes, I did bench bleed the cylinder. One outlet was perfect. The other one looked really good, but there were a couple very tiny bubbles left in. Could that be causing the issue? I bled the brakes for quite a while, so I figured any extra bubbles would come out the long way. Although now that I think of it, maybe there was something wrong with the new cylinder causing those tiny bubbles.

I think it’s another bad master cylinder.
The intermittent nature of it doesn’t seem possible otherwise.

Where did you get the replacement?
Some aftermarket parts are iffy quality.
I would have bought a new OEM part online for $131.

Were you told the problem was the master cylinder because a brake technician examined and diagnosed the problem, or because someone just assumed that was the problem based on what you told her/him?

Chances are it is a bad master cylinder, but I wouldn’t rule anything out without a thorough examination of the entire system.

And along with whitey…

You’re looking at brake pads with micrometers. i.e. seems only so much left for wear? You’re so good at looking at pads… replace the them!!..

I don’t have specs… but some cars by honda have anti skid valves in the rear or frigging rear end cross member valve that adjusts the rear end (like a mini shock absorber) can’t even see it unless it’s up in the air. good friend had one go bad. Not sure if you have one… but dumbest idea I ever saw. And you’d never see it until you squeezed the boot and fluid would come out.

Only mentioning it because of same squishy pedal action.