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Unread 01-05-2013, 09:41 PM   #1
AVR2
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Brakes losing pressure momentarily

Okay, so did my first lengthy drive after replacing both rear wheel cylinders, and after a while noticed that the brakes were momentarily losing pressure. Press the pedal and they start to bite and all feels normal, then at a random moment they'll lose pressure for just a fraction of a second and the pedal will drop slightly before they bite again. You can hear a sort of clunk noise when it happens, which I guess is the pistons releasing and immediately applying again.

This doesn't happen with every use of the pedal, but it's probably about 75-80% of the time. It's most noticeable with gentler brake applications, because you're not pressing the pedal with as much force.

The front calipers are less than a year old and the rear cylinders are brand-new, so I guess it's maybe the master cylinder or the power booster? Or is it just possible that there's a rogue air bubble somewhere in the system, even though I thought I bled the cylinders properly and got a really great pedal?

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Unread 01-06-2013, 02:50 PM   #2
ratmonkey
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sounds like maybe the install on the rear drums.
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Unread 01-06-2013, 09:35 PM   #3
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Quote:
Originally Posted by ratmonkey View Post
sounds like maybe the install on the rear drums.
What could I have done wrong that would result in a momentary loss of pressure (literally a fraction of a second), but not every time I brake?
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Unread 01-06-2013, 09:36 PM   #4
jtackerman28
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Does it have abs?
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Unread 01-06-2013, 09:39 PM   #5
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Yes. I'm not getting an ABS light, though. Are you thinking perhaps a false-positive from a wheel sensor?

Now that I come to think of it, it only happens when the ZJ is moving. I spent a few minutes or so just pressing and holding the brake while idling in park, and there was no pressure loss.
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Unread 01-06-2013, 09:52 PM   #6
jtackerman28
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Pull the abs fuse out and take it for a drive I bet it will clear it right up.
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Unread 01-06-2013, 09:58 PM   #7
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I'll give that a try tonight
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Unread 01-07-2013, 10:48 AM   #8
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Tried it - made no difference

I'm now definitely thinking about the master cylinder and/or the booster. The loss of pressure really is momentary, less than half a second, and it doesn't happen on every use of the pedal.
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Unread 01-07-2013, 02:16 PM   #9
Trick440
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Air still in the system?

If an air bubble got up in the line and you did only a few pumps during the bleed out, it might do like you discribed.
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Unread 01-07-2013, 09:37 PM   #10
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Originally Posted by Trick440 View Post
Air still in the system?

If an air bubble got up in the line and you did only a few pumps during the bleed out, it might do like you discribed.
I've been wondering that. I've got a set of speed bleeders in the mail, so I'll do a complete brake fluid flush when they get here. Weirdly, though, it only seems to happen when the Jeep is moving, although it's definitely not ABS-related. I rolled the front wheels up against the curb and kept pressing the pedal while idling in D, and the pressure loss didn't happen.
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Unread 01-08-2013, 05:18 AM   #11
mike9316
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Try the proportional valve, I lost my rear brakes altogether so it may not even be the same problem just throwing it out there
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Unread 01-08-2013, 05:59 AM   #12
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I did think about that, but there doesn't seem to be any kind of specific test for proper function of the valve. It can be bled, and that seems to be about it.
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Unread 01-28-2013, 12:31 PM   #13
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FWIW, finally got around to fixing this problem. I installed the speed bleeders and ran a couple of bottles of brake fluid through the system by pumping the pedal about 30 times at each corner. Now the brakes work completely normally. It must have been a trapped air bubble or similar causing intermittent problems.
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