Brakes Failed problems of the 2014 Toyota Sienna

On 8/2/18, while driving my 2014 Toyota Sienna van, bought new from a local dealer with appr. 89,000 miles and meticulously maintained for all service, I was driving on a city street when the brakes failed after pumping them three times. The brake failure caused my vehicle to rear-end the vehicle I was following. No injuries, but the van was totaled. No citation was issued by norman, ok. Police, accident report #2018-57682. Passengers in my van witnessed the brake failure.

At random and intermittently the brake pedal feel changes from firm to mushy with the pedal traveling to or near the floor during the stopping. Also at random and intermittently the car can be stopped with the brake pedal applied lightly to prevent car movement when the pedal suddenly slowly begins dropping and will go down to the floor. I have had no accident from either brake failure condition, but several times I felt ready to go into crash avoidance mode when there was the chance that this might also might be on the verge of loosing braking action. I had this checked by two Toyota shops but they said they found nothing wrong. I also called Toyota for assistance and a case number (1510051721) was assigned. The rep talked with both shops and lastly said he was going to talk to someone else and call back. He never called. When I drove from the 2nd Toyota shop, the brakes failed again and I went right back. The shop rep and I took the car out for a test drive and the brakes were ok. We switched driving and 3 blocks from the shop the brakes failed approaching a stop light. The light changed before we had to stop. At the shop, I stopped the car in drive and with my foot lightly on the pedal as in a normal wait for the stop light to change. After a short time, the brake failed and slowly dropped to the floor. This time the rep (who was still in the passenger seat) had the failure brought to his attention and he actually saw the pedal fall to the floor. He immediately took the car into the shop, but again the tech found no problem or failure. I asked the Toyota rep to have a engineer or brake expert decide what could cause this and precautionary replace the primary choice. This was refused. This van was bought new in January, 2015. The problem was first noticed in March. I now have only 6,400 miles on it.