While driving the break pedal rises and becomes tight car slows down by itself what is causing this

Tiny
LISA1515
  • MEMBER
  • 2005 HYUNDAI SONATA
  • 52,000 MILES
Put new breaks and master cylinder in no leaks
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Monday, September 12th, 2011 AT 12:59 AM

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Tiny
CARADIODOC
  • EXPERT
Brake fluid is trapped and can't get back to the master cylinder. When it happens again, loosen the mounting bolts for the master cylinder. If the brakes release, check the push rod to see if it's over-adjusted, or if the brake light switch is misadjusted and holding the brake pedal down. If they don't release, open the bleeder screws at the front calipers. If the brakes release there, check the rubber flex hoses.

Why did you replace the master cylinder? Any chance the fluid is contaminated with petroleum product?
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Monday, September 12th, 2011 AT 1:08 AM
Tiny
LISA1515
  • MEMBER
This happened 3 months ago and the mechanic put master cylinder in it was fine for 3 months nows it started again this week
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Monday, September 12th, 2011 AT 1:25 AM
Tiny
CARADIODOC
  • EXPERT
The first thing to check is the rubber bladder seal under the reservoir cap. If it's blown up and mushy, there' a petroleum-based contaminant in the brake fluid. The only proper repair for that is to replace every part in the hydraulic system that contains rubber parts that contacts the brake fluid, and all steel lines must be flushed and dried.

At 52,000 miles and six years old, it's very unlikely the master cylinder failed on its own. If it failed due to fluid contamination, that contamination will leach back out of other seals and hoses and recontaminate the new master cylinder. That typically takes a few weeks to a couple of months.

The way to prove or disprove this is to loosen the steel lines right at the master cylinder while the brakes are locked up. If they release, fluid contamination is the chief suspect. The misadjusted push rod or brake light switch that I mentioned previously wouldn't be okay for three months, then suddenly start causing a problem.
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Monday, September 12th, 2011 AT 5:28 AM

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