One problem related to service brakes has been reported for the 1999 Toyota Sienna. The most recently reported issues are listed below.
Driving on highway at 65 mph applied brakes and the pedal went to the floor -- no braking occurred. Happened once before on a city street. Seemingly random failure which the dealer says they have never heard of and have no knowledge of how to fix since it's intermittent. Nothing has been changed since everything is supposedly working correctly. The dealer wants to see the car again if the problem occurs again -- but the car might be totaled by then - -hopefully no one will be hurt.