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Toyota Matrix brake failure

I bought my 2010 Matrix XRS in October last year, with about 37k mls on it.
Last November the brake pedal got ‘soft’. I could brake by pumping, but with low pedal pressure it went down to the floor.
The dealership concluded it was a master cylinder failure, and promised to fix it for free even though the warranty had just expired.
Brakes were fine until April 2012 when exactly the same happened. Same diagnosis, same fix. No charge. Not 4 weeks later the same thing happened again. Now it was decided it was either mechanic error or part failure. Same fix.
Well, today Aug 27 the same thing again.
What is going on here? Four defective master cyls? A lemon car? Four human errors? Seems highly unlikely. Should I try another dealership? I’m ready to trade this car in even though I really like it and I had it less than a year.

It sure sounds like a batch of bad master cylinders, not really lemon worthy, but if it is still under warranty you are bound to get a good one sooner or later.

Thanks for your response. I was wondering if it could be an electrical or power brake system problem because the pedal only goes down when the engine is running. When the engine is off the pedal is fine.

Well, the pedal is just harder to press down with the engine off because the power assist feature isn’t alive at that point. It has little to do with the problem.
That’s quite bad, though, going through that many master cylinders. People can get hurt that way.

I think the dealer should look at the booster. It may be allowing some vacuum to pass to the rear seal of the master cylinder causing the damage. It would be worth the look anyway.

I’m inclined to agree with Keith.
And the booster assembly can be tested.

The booster/ master cylinder assembly will resemble the attached drawing. The front of the booster will be under high pressure from the diaphragm. There’s a seal at the rear of the MC too, but if the canister assembly seal isn’t holding the pressure from inside the canister, it could be forcing air past the MC seal and into the MC. The MC seal will be designed primarily to withstand the pressure from inside the MC chambers, the booster seal to withstand pressure from inside the canister. The pressure directions are opposite. If air is passing the booster seal, it’ll be pressuring the MC seal backwards.

These types of seals are generally designed to compress into a cavity when pressurized from a specific side. Backpressure them and they can fail.

http://tijil.org/Scion_Docs/05_tC_Shop_Manuals/Repair%20Info/Repair%20Manual/Brake/Brake%20Booster%20Assy/conponen.pdf

http://tijil.org/Scion_Docs/Scion_06_misc_docs/2007sciontc_ncf%20Folder/brake.pdf