I have done those tests, not necessarily exactly in that order, and the brakes pass. I have downloaded that manual now, so I can do it in order, and exactly.

I did not disconnect the booster vacuum line, I will try that tomorrow if I have time to work on the car again. However, if the booster check valve leaks, it usually means you get intermittent power assist or none at all, because the brake booster will not hold vacuum. I clearly get power assist when the engine is running, the difference is easy to tell.

With the engine running, driving or full stop at idle, the pedal will gradually sink to the floor unless I give it 2-3 quick pumps first. Since there is no loss of fluid, I would have said that means fluid is leaking past the seals inside the MC. The pedal sinks gradually with an even pressure. Does not feel like air in the lines.

I will see if I can get the banjo bolts off, too. Thanks again for the tips.

 

I have the same problem in my honda civic 1994. I replaced rotors, pads, rebuild calipers, change brake fluid, bleed the lines at all wheels couple of times, checked for leaks, everythings good. So the only thing it could be is the booster or master cylinder. From your thread I'm leaning towards the MC but I'm not 100% sure. I don't want to spend $200 on new MC and then find out it wasn't it. Could you please post the answer to this problem when you resolve it? Thanx.

 

No luck so far.

I replaced the MC with a remanufactured one (Cardone) from a local part store. Simple job, really. Just one of the brake line fittings hard to get to. Cost was about $50 after the core. Bled all four corners again (I am on my second quart of fluid now(!), but - still have the sinking brake pedal.

I went over all the brake lines I can see, and there is no visible leak, and no loss of fluid. Brakes work fine, but the pedal action is hardly 100%, and I would hate to make a panic stop this way from hwy speed.

At idle, the rpm will dip slightly when the pedal is pressed, then rise to about 1K for about a second when the pedal is released. I disconnected the vacuum line at the booster, and there was plenty of suction. Power assist is good. The car does not have to be driven until I get everything working right, so I will keep at it when I have time. If necessary I will replace the MC again. I'll post a solution if I find one.

 

Sorry chief, sounds like the vacuum booster. I did nearly the same thing as you replacing the master cylinder first only to find out that wasn't it.

Real PIA working under the steering wheel too. Not impossible, just a pretzel postion to have to work in. Whatever you do, DO NOT change the pedal throw on the new vacuum booster. I did, had to do over.

 

Could you explain that a little bit more.

 

Thanx. I appreciate your reply. Can't wait to find out what the problem is. I have been driving with those sinking brakes for over a year. Its not very safe, but I kinda gave up and got use to it.

 

Sure. The brake pedal swing arm attaches to the vacuum booster. the u bracket on the vacuum booster is adjustable.

I adjusted mine too far out and the brakes were engaging without depressing the pedal.

I found out later that I should not have had to adjust it at all.

Live and learn....

 

I would assume that if the booster does not work due to a vacuum leak inside, the pedal would be hard to press. There is plenty of power assist. The pedal effort with engine running is normal, but with even moderate pressure, the pedal slowly sinks all the way to the floor. Similar to what it would feel like with a bleed screw open.

Normally if the booster fails, the brake pedal becomes hard to press, because there is little or no power assist from the booster.

 

Whatever you say Chief. Mine had the sinking pedal. New Vacuum Booster fixed it.

 

Ok, point taken and thanks. I will go over the brake system one more time in the next few days, if I find nothing, the booster goes next. Again, thanks to all who replied!