Brake pedal going to floor, but only sometimes?

[TJeeper]

Recently I have been having a problem with the brake pedal going to the floor on my TJ, but it only happens occasionally. I bled the brake lines, checked for leaks, nothing. It has been suggested the power brake booster has gone bad. Does this sound right?

Thanks-

[Robert J. Yates]

That is exactly what the master cylinder in my Suburban did.

[TJeeper]

So you are saying it might be the master cylinder? I had a shop do some work on my Jeep recently and someone accidently cut a brake line. They assured me the brake fluid did not empty out of the reservoir, but it sounds to me there might be air in the master cylinder.

[mrblaine]

Originally posted by TJeeper
So you are saying it might be the master cylinder? I had a shop do some work on my Jeep recently and someone accidently cut a brake line. They assured me the brake fluid did not empty out of the reservoir, but it sounds to me there might be air in the master cylinder.

You are in CO, can you describe the circumstances under which it goes to the floor?

What I am looking for is a correlation between elevation and temperature.

Hot days, slow speed and higher altitude will let your brake fluid boil sooner. That would explain why it only does it sometimes.

Other than that, replace the master. That would be rare for the TJ master to go out though.

[Robert J. Yates]

Well,
I am not saying for sure its your MC - just passing along an experience that was not to far in the past for me. Ocassionally the pedal would go to the floor and slowly come back up and I never lost my brakes. Finally, I went to the local brake guy and he tested it while I was at work. I was told the MC was bad and needed to be replaced. I replaced it and have not had the problem reoccur so I assume he was right and it was the MC.

Seeing how you had an issue with a shop, I would go back to them, explain your problem and make it clear that you are not happy. Or OTH, you can pull and bench bleed the master yourself which is what will need to be done if they drained it of fluid when they cut the line. You will then have to bleed the entire system after you reinstall the MC.

[TJeeper]

Blaine-

This is occuring at start up. The temperature was about 50F, so I don't think it is heat or altitude related.

Robert-

The work was done at a friend's shop, and the price was heavily discounted. Yes I could bring it back and say fix it, but I would rather try and figure it out on my own for now. My friend suggested the brake booster, but that seems doesn't seem right to me (not like I am an expert mechanic).

If there was air in the MC, would this be evident when bleeding the lines? We did them twice to make sure that wasn't the problem. Or if there was air in the MC and the brake pedal was pressed, could the MC have been damaged in a way that the brakes would have pressure most of the time, but occasionally then lose pressure and allow the brake pedal to be pressed all the way to the floor?

I guess I will start by pulling the MC and bench bleeding it, then the lines, and go from there.

[Bruce David]

Your Master cylinder is toasted.
Think about this, it has happened to me before.
Every time you apply the brakes the peddle moves the same amount.
Sometimes a little farther if your hammering them, but for the most part
the pedal stops in the same place.
Well, right behind where the lip seal/o-ring stop, deposits build up.

Now comes along Mr. torch or Mr. Sawzall and cut your line.
No biggie, Mr. Mechanic splices the line back together.
and bleeds the system.

This is where the damage to the MC occurs.
When bleeding the brake pedal goes all the way to the floor
and all the little crusty deposits, that are right where the MC usually stops,
get scraped by the lip seal/o-ring and chew them up.

[TJeeper]

So this isn't the fault of the shop, this would have happened the next time I bled the brakes- right? Although I don't quite understand if the seal in the MC is ruined, why I have normal braking pressure most of the time, but say 1 out of 10 times it goes to the floor.

[TJeeper]

Got it figured out, thought I would let everyone know about my stupid mistake. First, I changed the MC out, took the TJ out for a drive, the pedal would still go the floor occasionally. I started doing circles in the cul-de-sac in front of my house (yeah, my neighbors were watching me) and realized that when I made a right hand turn, the brakes would go out, but not a left hand turn. I pondered that for a few hours when I realized I should take a look at the front brakes with the steering wheel turned. Sure enough, I noticed a shiny ding on my new shocks, the caliper was hitting it.

I just put in a HP30 that the previous owner had modified the shock mounts by moving them down. Looked like a good idea, but now I have four washers in my left steering stop to compensate.