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The '99 Sienna and the Phantom Brake Problem

The day after I left our 1999 Toy Sienna (155,000 mi) at our vacation place for my wife to use, she and my daughter both swear that the brakes failed completely during a drive. (pedal to the floor, no effect) Coasted to shoulder, no one hurt, no damage. The best brake mechanic in the area could not find a single thing wrong after inspecting system and pulling all the wheels. Never ever happened again!! What’s up with that???

This one almost HAS to be a master cylinder failure. If it is, it’ll fail again. Personally, with 155,000 miles on the original and my family riding in the seats, I’d change it out just as a precautionary measure.

The one thing that would change my mind would be if it failed at the bottom of a long descent. Then I’d suspect the fluid overheated. In that case It’d have the fluid changed out and teach them how to downshift and to modulate the brakes.

Ditto to MB’s theory.

This is either a case of a failing master cylinder, or it is a case of overheated brakes from driving on a steep downgrade without downshifting. Is this vacation home in the mountains?

Even if it is not listed in the mfr’s maintenance schedule, it is very wise to have the brake fluid changed every 3 years. Since brake fluid is hygroscopic (absorbs moisture from the air), over time the fluid becomes diluted with water. When brakes become very hot–as when driving on a long, steep downgrade without downshifting–the diluted brake fluid will actually boil. The boiling brake fluid prevents the hydraulic system from being able to exert any force on the brake calipers, and as a result you have no ability to stop until the brakes cool down.

Given the age of this vehicle, I strongly suggest that you have the master cylinder replaced and have the brake system flushed at the same time.