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I have a 2012 Jeep Wrangler and I've experienced something very scary three times in the past couple of years.

I was driving up to an intersection and put my foot on the brake and the brake pedal went completely to the floor with zero braking applied. I immediately pulled my foot back and pressed again and the brakes worked perfectly.

The first time this happened ~year ago and I thought maybe I just imagined it, but it happened again two times in the past few months.

The really bizarre thing is that after these incidents the brakes work perfectly. There is no play in the brakes, I don't need to pump the brakes like there is air in the line. It really appears to be a one off problem.

I looked online and found several things people proposed as the problem:

  • bad master cylinder
  • bad brake booster
  • air bubble in abs system

The master cylinder seems unlikely because the brakes work perfectly after each incident. If the brake booster were having problems, I would imagine that instead of the pedal going to the floor it would be really hard to press and I would still have some breaking power. I don't think the ABS system engaged, so I don't see how it could be the bubble either.

Based on these, I suspect maybe it is some random problem with the ABS system? I have had random instances since I bought the car where the ABS error light will start blinking on and off while driving.

I checked the brake fluid levels and it is somewhere between min and max. I haven't had to top it off since I can remember, so I don't think I'm losing fluid.

Any ideas on what could be causing this issue?

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If this happens after you've been braking quite a bit shortly before the incident, it could be boiling brake fluid. Brake fluid is hygroscopic and will absorb moisture from the air. When the moisture gets too high, the heat from braking will cause the water in the fluid to boil. This causes air in the system bringing on this condition. Brake fluid should always be flushed at the recommended interval stated in the manual to prevent this. Note this moisture in the fluid also contributes to rapid corrosion to brake components.

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  • Minimum recommendations to replace brake fluid is after replacing bake pads in disc brakes. Usually a flush and bleed of all four wheels. Moisture absorption occurs as master cylinders of most vehicles are vented. As brakes wear, brake fluid level drops and pulls in air. Your master cylinder is worn. You're flirting with disaster the longer you delay this.
    – F Dryer
    Nov 8 '21 at 16:58
  • It's "hygroscopic"
    – mike65535
    Dec 6 '21 at 13:33
  • Thanks. Fixed spelling
    – Jupiter
    Dec 6 '21 at 14:08

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