Brakes Intermittently drop.

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DL563

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Hello everyone, I have a 1989 bronco with the 5.8l engine I've been having an issue that I haven't been able to quite figure out. While I'm driving the brakes work very well and stop fast but in certain scenarios the brake pedal completely drops to the floor without warning then eventually comes back up. It's mainly happened while backing out of my driveway (on a steep incline) or following hard acceleration. I had changed the master cylinder and brake booster about a year and a half ago due to another issue so I would hope they're good, and following this issue I just changed the rabs valve. I don't know what else to check at this point theirs no visible leaks and it being intermittent has got me scratching my head.
 

Tiha

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Pedal dropping to the floor is pretty much always the master cylinder.

I know it doesn't seem like they should fail that fast but they do, replacement parts are poor quality.

Just did one on my kids vehicle Friday night. It was less than a year old. Was leaking fluid out the back between the booster and MC.
 

Motech

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I agree with @Tiha about to brake master cylinder almost always being the cause of an intermittent pedal drop. I would say warranty your master first.

That said, those RWABS systems can cause some real headache inducing issues. Usually though, they cause a constant low pedal that will pump up every time. So as long as you're not getting that, I would focus on that master cylinder. Then, if you still have issues, it might be time to look at that system.
 

Motech

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BTW, those RWABS systems can be a real bear to bleed out. So if you still have problems after replacing and bleeding the master cylinder, post back up here and I'll post up some technical service bulletins about it.
 

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