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Master cylinder failure, what could be causing undue pressure on the Master cylinder and blowing the seals?

Tiny
JEFF HERMAN
  • MEMBER
  • 2006 TOYOTA HIGHLANDER
  • 2.4L
  • 4 CYL
  • AUTOMATIC
  • 133,000 MILES
In 133,000 miles this vehicle has been through four master cylinders and two brake boosters. The first two replacement master's I believe were AC/Delco and now another genuine Toyota less than a month ago which is already failing. Twice there was fluid in the booster. Calipers are not sticking and there are no apparent leaks. I'm trying to help this owner, but the vehicle is in Florida, and I live in Wisconsin. Isn't there some sort of distribution valve that might be causing undue pressure on the master and blowing the seals? I know Toyota had a TSB out about using genuine Toyota brake fluid that contains certain lubricants. What are your thoughts on this issue? Obviously, her mechanic can't figure it out.
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Wednesday, February 7th, 2024 AT 8:20 AM

4 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 42,949 POSTS
Yep, the brake fluid can cause the seals to go out, also if the ABS system is used constantly like they have an ABS wheel speed sensor that is not reading correctly it will cause extra pressure on the bake master, can you ask if the brake pedal vibrates when the brake pedal is depressed?
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Thursday, February 8th, 2024 AT 10:27 AM
Tiny
JEFF HERMAN
  • MEMBER
  • 394 POSTS
No vibration, pulsating, or ABS light on.
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Thursday, February 8th, 2024 AT 11:05 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 42,949 POSTS
Then it must be the brake fluid, there is nothing I can think of that would cause this kind of problem except for extreme usage.
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Friday, February 9th, 2024 AT 9:11 AM
Tiny
JEFF HERMAN
  • MEMBER
  • 394 POSTS
Yes it's baffling. 4 masters and 2 boosters since 2020. The last one factory Toyota about a month ago with proper fluid.
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Friday, February 9th, 2024 AT 9:14 AM

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