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Old 11-27-2005, 11:15 PM
Jerry Bransford Jerry Bransford is offline
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How can just changing brake pads cause a low pedal?

Changed the pads in my wife's JGC Saturday (biggest PITA pad-change I've ever done) and the brake pedal went to the floor when I started the engine and stepped on the pedal. The stopping distance is also now a bit longer unless I press harder than before on the brake pedal.

Brake fluid overflowed the reservoir while pushing the pads apart to place the caliper assembly back over the rotor, as it often will. Nothing unusual there. But after test driving the Jeep, the fluid level was still up near the top as if it hadn't lost any brake fluid at all. I'd have to guess a lot more brake fluid overflowed than there is now room for in the reservoir, which indicates it's full for all intents and purposes.

Ok, I plan to get the brake lines bled & power flushed tomorrow since that's the only thing I can think of, that air must have gotten into the lines somehow. But how? The brake lines aren't leaking and I didn't disconnect any of them.

Big mystery for a guy who is just competent enough to usually be able to change brake pads/rotors without too much thought needed.

Any ideas? Thanks!
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Old 11-28-2005, 07:13 AM
mrblaine mrblaine is offline
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Re: How can just changing brake pads cause a low pedal?

Quote:
Originally posted by Jerry Bransford
Changed the pads in my wife's JGC Saturday (biggest PITA pad-change I've ever done) and the brake pedal went to the floor when I started the engine and stepped on the pedal. The stopping distance is also now a bit longer unless I press harder than before on the brake pedal.

Brake fluid overflowed the reservoir while pushing the pads apart to place the caliper assembly back over the rotor, as it often will. Nothing unusual there. But after test driving the Jeep, the fluid level was still up near the top as if it hadn't lost any brake fluid at all. I'd have to guess a lot more brake fluid overflowed than there is now room for in the reservoir, which indicates it's full for all intents and purposes.

Ok, I plan to get the brake lines bled & power flushed tomorrow since that's the only thing I can think of, that air must have gotten into the lines somehow. But how? The brake lines aren't leaking and I didn't disconnect any of them.

Big mystery for a guy who is just competent enough to usually be able to change brake pads/rotors without too much thought needed.

Any ideas? Thanks!
If you were able to get the pads over the rotors, you should have had to pump the pedal several times to move the pistons back out after the re-install.

The pads ride in light contact with the rotor during normal usage and need to be that way to work.

The master overflowed because you added fluid during the normal wear cycle of the pads. You got a bit worried and saw that it wasn't topped off or at least someone did and topped it off. The level drops as the pads wear. It normally won't drop below the minimum level and will come back up to the top when new pads are installed.

You also didn't mention whether or not you bedded the new pads on re-install. If you didn't, you need to.

The reason they worked worse initially is that as the rotor wears slightly, it gets minute grooves in it and is not perfectly flat. Bedding the brake pads will bring all of the friction material into cotact with the rotor and give the most contact.
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Old 11-28-2005, 08:33 AM
Jerry Bransford Jerry Bransford is offline
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Well there ya go, those answers make sense Blaine. Yep I went through a bedding/seating/break-in process with them, they are PF carbon metallic pads. Thanks a bunch Blaine, that made me feel better.

I'll still get the brakelines power flushed today, as much to make my wife feel better as anything.
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Old 11-28-2005, 09:19 AM
NAILER341 NAILER341 is offline
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it is a good idea to crack the bleeder open rather than forcing the old fluid back up the line as well. this will eliminate the overflowing of the reservoir as well.
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Old 11-28-2005, 10:17 AM
mrblaine mrblaine is offline
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Quote:
Originally posted by NAILER341
it is a good idea to crack the bleeder open rather than forcing the old fluid back up the line as well. this will eliminate the overflowing of the reservoir as well.
The best way to prevent overflowing is to not add fluid unless it's below the "add" line on the master cylinder reservoir.

They are sized in a manner that let's you maintain an adequate fluid level throughout the pad wear range and when new pads are installed, the fluid will rise to the full mark.
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Old 11-28-2005, 10:27 AM
quadna71 quadna71 is offline
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Quote:
Originally posted by mrblaine
The best way to prevent overflowing is to not add fluid unless it's below the "add" line on the master cylinder reservoir.

They are sized in a manner that let's you maintain an adequate fluid level throughout the pad wear range and when new pads are installed, the fluid will rise to the full mark.
that's funny....cause i always added fluid through the life the pads. hmph - you learn something new every day!
thanks MrB
chris
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Old 11-28-2005, 10:52 AM
mrblaine mrblaine is offline
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Quote:
Originally posted by quadna71
that's funny....cause i always added fluid through the life the pads. hmph - you learn something new every day!
thanks MrB
chris
It's a sealed system, where does the fluid go that you need to add more?
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Old 12-04-2005, 10:25 AM
quadna71 quadna71 is offline
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well, i knew that fluid wasn't going anywhere (15+ years in aviation pneudraulics ) but was always just worried about lower fluid levels maybe slurping in some air when off camber. i never really took the time to see how low it would have to be before pulling in air. it makes complete sense what you're saying though...just never really followed the thought process through.
chris
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