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Where'd my pedal go?!

TRSCobra

NAXJA Forum User
Location
Gig Harbor, WA
I was driving off road a few weeks ago in my '98 XJ and I went up a steep hillclimb. I got turned around in a maze of trails, and came to what I thought was the top of the same hill. I didn't take a good look since I thought I had already gone up it. Well, it was a different hill, and there was a large section washed out. I found this out after my front end was down in the ditch so bad that I was close to rolling end over end (i know this sounds impossible, I was diagnally descending this hill with an embankment stopping my front wheels.)
My brake pedal was going to the floor. I thought that I blew a wheel cylinder or a brake line at the worst possible time. The wheels were stuck to the extreme left, with the weight of the jeep resting on them. And it looked as if the back of my left caliper was touching my axlehousing! I looked all over and found no leaks. The brake light never came on, either.

When I finally reached level ground, (had to get pulled back afew feet and try it again) my pedal returned to normal, and it feels as if nothing ever happened, theres no air in the system.
Is it possible that the angle I was at was starving my master cylinder of fluid, or would the proportioning valve have something to do with that?? Or could it have been the pressure on my caliper that did it? Has anyone encountered this before?
 
my '98 does that in panic pushes of the pedal, so you are not alone.
 
I imagine it's possible the master cylinder was starved, but I'd expect a warning light if that happened. Besides, if you look at the reservoir, you'll see that it would have to be practically upside-down before it could starve. It is, after all, designed to work reliably even under high-G deceleration.

Another possibility (and a definite possibility for "2offroad" posting above) is a worn master cylinder. The cups in the master cylinder are designed to push outward under pressure to increase sealing, but sometimes when they wear they will fail to open out enough. Usually this will happen when you push the pedal slowly and gently, and show less when you push it fast and hard, or pump it, but it isn't always predictable. I've had MC failures where the pedal would just occasionally drop, then behave perfectly for the next week. I'd have a really good look at the master cylinder anyway, just in case. A good start is to start the engine, and then push slowly and medium-firmly down on the pedal to see if it keeps sinking.
 
Mine only does it on steep climbs, when I'm tring to keep from sliding back off the ledge. The brakes hold good only the pedal goes to the floor. I think it is a booster issue myself, not problem enough to look into.
 
2offroad said:
Mine only does it on steep climbs, when I'm tring to keep from sliding back off the ledge. The brakes hold good only the pedal goes to the floor. I think it is a booster issue myself, not problem enough to look into.

A bad booster will NOT let your pedal go to the floor. The booster will only provide boost, and if it is bad it will make the pedal harder not softer. If your pedal is going to the floor you have a hydraulic issue. If you don't have a leak, then you almost certainly have a master cylinder on its way out. Remember that this is a dual master cylinder, and that means that if either the front or rear portion fails it will go very near to the floor but still hold, but it will NOT stop you reliably at speed. In short, this is a brake failure, and I would strongly advise that you try to troubleshoot it before it pops up on the highway.
 
My jeep only has 63k miles on it? And this only happened that once, and it immediately corrected itself once I leveled out.
 
2offroad said:
Mine only does it on steep climbs, when I'm tring to keep from sliding back off the ledge. The brakes hold good only the pedal goes to the floor. I think it is a booster issue myself, not problem enough to look into.

NOTE TO SELF:
NEVER follow 2offroad up a hill.

Rev
 
It has been about 20 years since I was a mechanic, so I have forgot a lot of details. Please correct me if I am wrong. On a vacuum booster the rod that the pedal is attached to presses on a diagram and another diagram pushes in on the master cylinder, if you have a booster failure the rods will touch and you will have full brakes just at the bottom of the travel. If a master cylinder fails you won’t have full brakes because the fluid will leak back around the seals and the brakes won’t hold. On mine the brakes hold only at the bottom of the travel so that leads me to believe that it is a booster issue, also it only happens in a nose up or down attitude, never on the road or under full panic stop
 
2offroad said:
It has been about 20 years since I was a mechanic, so I have forgot a lot of details. Please correct me if I am wrong. On a vacuum booster the rod that the pedal is attached to presses on a diagram and another diagram pushes in on the master cylinder, if you have a booster failure the rods will touch and you will have full brakes just at the bottom of the travel. If a master cylinder fails you won’t have full brakes because the fluid will leak back around the seals and the brakes won’t hold. On mine the brakes hold only at the bottom of the travel so that leads me to believe that it is a booster issue, also it only happens in a nose up or down attitude, never on the road or under full panic stop

I don't think that is quite right. There's no air gap between the rods, at least on the current crop of boosters. You can simulate booster failure by simply pushing the pedal a couple of times when the vehicle is not running, or criimping off the vacuum line when it is. You will see that you get a hard pedal, with much multiplied effort. If your booster is working, when you push the pedal and start the engine, the pedal will sink. There is a difference between "holding" and "stopping." With modern dual master cylinders, failure of one brake circuit will cause the pedal to sink nearly to the bottom before the second circuit kicks in (there's a spring between the two pistons inside the MC as well as some tricky hydraulic circuitry). Either circuit will almost certainly hold, but this does not mean it will stop you safely at speed. Because of the design of the master cylinder cups, it will usually work best in panic stops, and hard applications, and fail first under steady pressure.

In addition, if the failure you get is dependent on nose up or down attitude, it almost certainly is hydraulic rather than pneumatic. The booster, after all, has no fluid in it, so it will work the same even upside-down. It sounds much more as if there is a fluid starvation issue here. I haven't gone out and put my Jeep over an embankment to see how steep it must be to starve one of the brake circuits, but I suppose this is possible. However, I find it unlikely that this would happen normally, given the way the reservoir is designed, unless the fluid level is already low or something else is wrong. I still think you'd do well to look very carefully at your system and make sure there's no master cylinder failure lurking in your future. New master cylinders are relatively cheap compared to some of the possible consequences of brake loss.

I say this, by the way, not as a flame, etc. I've driven vehicles over the years with all sorts of bad brakes, bad boosters, iffy master cylinders, rotten lines, etc. My first Jeep truck had frozen adjusters that required two pushes of the pedal to stop. I once drove a CJ-5 for a week with nothing but the handbrake, and for another month with a rubber hose on the rusted-out line. For the last two years I had my Scout, the rear brake circuit was crimped off (those brakes sucked so badly anyway, it never felt the difference). And on and on...If you live in New England as long as I have, you'll probably have experienced every variation of brake failure known at least once. I just think that if you do have iffy brakes you should try to figure out exactly what's wrong with them so you're ready.
 
Matthew Currie said:
I say this, by the way, not as a flame, etc. I just think that if you do have iffy brakes you should try to figure out exactly what's wrong with them so you're ready.
no flame taken, thats is why I said it's been a long time since I have done diagnostic work on brakes, you maybe right on the booster and fluid loss. Mine hold 95% offroad even under light pressure, 100% of the time onroad, so I was thinking maybe booster acting up, fluid in the booster or some other factor causing the pedal to go to the floor but the brakes holding
 
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