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

2006 TOYOTA HIGHLANDER
133,000 MILES • 2.4L • 4 CYL • AUTOMATIC
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JEFF HERMAN
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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.
Feb 7, 2024 at 8:20 AM
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STRAILER
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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?
Feb 8, 2024 at 10:27 AM
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JEFF HERMAN
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No vibration, pulsating, or ABS light on.
Feb 8, 2024 at 11:05 AM
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STRAILER
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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.
Feb 9, 2024 at 9:11 AM
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JEFF HERMAN
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Yes it's baffling. 4 masters and 2 boosters since 2020. The last one factory Toyota about a month ago with proper fluid.
Feb 9, 2024 at 9:14 AM
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STRAILER
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anything?
May 10, 2024 at 5:36 PM
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JEFF HERMAN
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I suggested she put some dye in the system because if the vacuum line to the booster is dry there is most likely a leak somewhere. I haven't heard anything. Unfortunately she is in Florida and I am in Wisconsin. I'll keep you posted.
May 10, 2024 at 9:06 PM
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STRAILER
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sounds good, have a nice weekend
May 11, 2024 at 1:11 PM