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Unread 01-27-2011, 08:33 AM   #1
jake7893
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Weird braking problem. Just need advice

Yesterday my brakes almost completely went out. I was on the highway (daily commute now) as I was on the off ramp my brake pedal went almost completely to the floor. I can pump them up and build pressure, and it helps stop. I dont' know why it only does it on the highway? Is it becuase I rarely use them on the highway? My fluid level was a little low, bt I filled it up. What should I do/check for? Any ideas?

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Unread 01-27-2011, 08:35 AM   #2
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98 TJ, 2" BB
31" BFG AT's
4 cylinder
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Unread 01-27-2011, 09:12 AM   #3
Piratejonh
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No leaking anywhere?
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Unread 01-27-2011, 09:17 AM   #4
jake7893
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No, the fluid level was just a tiny bit low, but I did fill it up too high now. I don't think the fluid level is the problem though, because it seems to lose pressure as I'm moving. Very weird.
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Unread 01-27-2011, 09:33 AM   #5
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it's a closed hydraulic system. if the pedal goes to the floor it's from a leak, rear brakes way out of adjustment, master cylinder bypassing or pedal fade from excessive heat in the system. first check that all the wheels are free, jack it up and make sure you can spin the wheels by hand. next check for leaks including pulling off the rear drums and checking the rear wheel cylinders. if no leaks are found try flushing out the system with fresh fluid.
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Unread 01-27-2011, 09:44 AM   #6
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I had a similar problem, I ended up siphoning my resivor fluid which was dirty and flushing around 32OZ of fluid through the entire system. they recommend a flush at 32000 and im at 65000. You also might have small bubbles which flushing will get rid of. I bought the $7 one man bleeder at autozone and it worked fine. Just every 6 brake pedal pumps I had to empty the small bottle into a container. Start with the back right then back left etc. with front.
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Unread 01-27-2011, 09:51 AM   #7
jake7893
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I'll start by jacking it up and checking, then running new fluid through the lines, come to think of it I know the lines haven't been completely flushed in about 30,000 miles. I've bled them, but not completely flushed, and I don't know when the PO had them last flushed. Thanks guys, I hope this helps. I don't understand how old brake fluid would make the pedal spongy sometimes. Weird.
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Unread 01-27-2011, 10:08 AM   #8
mrblaine
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Originally Posted by jake7893 View Post
I don't understand how old brake fluid would make the pedal spongy sometimes. Weird.
Brake fluid by it's nature inherently absorbs water. It does this to isolate it away from your brake components to stop or slow down corrosion.

If you read the label, there will be two values for the Dry boiling point and the Wet boiling point of the fluid. The wet is when the fluid will boil at roughly 8% water content and it's significantly lower than the dry point.

What happens is the lower boiling point will let the fluid boil and create pockets of compressibility. That's pretty easy to do if you have a caliper dragging at highway speed and a high water content in the fluid.

As the system cools back down, it will come back up to snuff and you'll think nothing is wrong until it happens again.

I am not saying this is your problem, but only an explanation of how it can happen.
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Unread 01-27-2011, 10:21 AM   #9
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Thank you very much, that makes perfect sense. I had a caliper dragging a while back, and lots of brake problems with my Jeep. I bet it has boiled/been boiling and created the pockets. I'll still check to see if anything is hung up now, but hopefully the flush will work. Thanks all!
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Unread 01-27-2011, 10:29 AM   #10
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Thank you very much, that makes perfect sense. I had a caliper dragging a while back, and lots of brake problems with my Jeep. I bet it has boiled/been boiling and created the pockets. I'll still check to see if anything is hung up now, but hopefully the flush will work. Thanks all!
They go away when it cools back down to below the boiling point.
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Unread 01-27-2011, 10:45 AM   #11
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How would air bubbles get out of a sealed system? I can't see them being compressed back into the liquid
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Unread 01-27-2011, 10:59 AM   #12
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How would air bubbles get out of a sealed system? I can't see them being compressed back into the liquid
They aren't air bubbles, they are pockets of steam that condense back to liquid when they cool.
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Unread 01-27-2011, 11:02 AM   #13
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Ohh.. I would think my brake fluid wouldn't be boiling at highway speed unless I do have a dragging caliper. It works just fine in the city though. I guess I'll have to start tinkering and figure out what's going on.
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Unread 01-27-2011, 11:05 AM   #14
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Ohh.. I would think my brake fluid wouldn't be boiling at highway speed unless I do have a dragging caliper. It works just fine in the city though. I guess I'll have to start tinkering and figure out what's going on.
I already said that it MAY NOT be your problem, I was just giving you a scenario of how that could happen.
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Unread 01-27-2011, 11:10 AM   #15
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Thanks for explaining, I'm just brainstorming online of what it could possibly be. I'm just hoping for the cheapest/easiest fix... which it usually never is anyway. The brake system is the one area in the Jeep that I really lack knowledge. I've fixed rear main seals before, which leaked and caused my back brakes to not work at all, but I was mainly just following instructions of the mechanic who was guiding me. Thanks again,
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