Need some tips on diagnosing Brakes...

I have a recently purchased 96 Bronco, and the brakes are behaving strangely.

Every once in a while the pedal will have excessive travel. Not quite to the floor, but almost, and enough to scare you. As soon as you pump the pedal, the pressure comes back. The master cylinder was replaced not that long ago by the previous owner, and by appearances looks quite new. I have adjusted the rears, and put new pads on the front. None of the wheel cylinders appear to have any leaks, nor is there any leaking fluid from the Master or ABS pump.

The brakes stop fine most of the time, and even when the pedal goes too far, it does still stop the truck.

Any thoughts on what could be causing this? I'm guessing it has to be the master cylinder, but i've never heard of one going out in this way. I thought once they fail, they fail completely and you know it right away.

Thanks!

 

New MC but the pedal goes away intermittently. Was it bled properly? Air that far up the line will take forever to get all the way through the system.

 

I assume it was.. It didn't start doing it right away when i bought it, and it doesn't do it all the time. I Can certainly go ahead and bleed it out again, but wouldn't air in the lines cause a spongy pedal all the time, not just intermittently?

 

Yes, you are right. Might check vacuum supply to the brake booster. A cracked line might cause intermittent issues.

 

good suggestion, i'll try that.

but wouldn't a vacuum leak to the booster cause an occasional hard pedal?

 

Unless you are leaking more than you are getting vacuum in which case the pedal would always be hard and suddenly feel soft with the vacuum assist. Other than an intermittent leak or a seeping cylinder, there isn't much else in the braking system to deal with. Does the antilock system ever engage? I only ask because you maybe getting errant faults in the ABS.

 

I'd start with vacuum as well. I had a friend who's jalopy had two symptoms that would swap back and forth: sometimes it would run bad while other times it would have almost no brakes. It turned out he had both a bad MC and an intermittent vacuum leak. When the vacuum was leaking it ran poorly but you didn't notice the brakes were not what they should have been because you weren't getting any boost.

 

I have the same strange pedal fade. The first pedal push to stop is perfect. If you push firmly on the pedal only once and maintain that push/pressure and come to a stop and hold. There is absolutely no pedal fade. If, for example, you are stopping gradually and just slightly releasing pedal pressure and then adding pedal pressure you will get more and more pedal fade each reapplication of pedal pressure.(like pulling into a parking spot in a parking lot) The amount of fade is not consistant and can really surprise you. If, at the point of fade, you do a full pump of the brake you will get the first pedal push scenario again (perfect pedal)

I wish this thread had been complete with an I FIXED IT BY DOING XX.

I am posting this iin case someone who knows returns.

 

sorry about that... definitely should have replied back to report the solution.

I started out with the intent to flush and bleed all the brake lines.

The bleed nipple was so frozen into the driver side caliper, that I had to take it off the truck and work on it on the bench to force it out. Since I had to go to the trouble of removing it, I decided to just go ahead and replace them both since they are cheap, and it's just as much work to put new ones on as it was to replace the old ones.

After replacing both, I completed the flush and bleed at all four corners, and the problem went away. So, either something was wrong with one of the calipers, or there was air in the system somewhere.

Brakes are great now.

 

Thanks for your quick response!!!

I do have a couple of questions if you don't mind.

Does the description of my problem sound "alot" or "kind of" like your experience?

When you bled the brakes, did you do it the standard way. That is, with the ignition key off.

I ask the last question because during my researching of the problem on this site I remember a statement that indicated due to the ABS/HCU units bleeding needed to be done either "KEY ON" or "ENGINE RUNNING".

Just wondering if that was true.

Thanks again!
Jccastl

 

only similar, not exact.

My pedal was unpredictably soft. Not so much would fade, just that it would work most of the time, but then occasionally would go very low. It would still stop, but felt like there was no pressure in the system. A quick pump would restore pressure and height to the pedal.

When I did the flush, I did not have the key on.

 

By fade I mean losses pedal height. Your description reminded me of my initial feeling about the problem, but since I HAD to drive it that way I spent a bunch of time characterising the problem and learning exactly how to reproduce it consistantly so I could avoid it and KNOW I wasn't going to just loose brakes all together.

Anyway, my brakes also will stop the bronco even when pedal gets very low. It's just a little unnerving to have the place/height of pedal randomly change relative to brake engagement.

Thanks again for your quick response.
WOW, brake bleeding in a snow storm with the wife pumping. GOTTA LOVE IT!

jccastl