2004 Toyota Sienna soft brake woes

Been a mechanic for 20 years now. Wife has a 2004 Toyota Sienna XLE Limited AWD. This car has been a huge pain in the neck for me the entire time we have owned it. Great car, sucks to work on.

A year ago, she would sit at a stop light and after a bit she would notice that the car would start to creep forward. I verified this, then diagnosed it as a bad master cylinder. Pads and rotors were new, all hardware clean and greased.

After a month, the car started exhibiting the same symptoms with the new master cylinder. I warrantied it out and replaced it.

After about another month, same thing. This time I went and bought the Toyota computer system and bled the brakes via OBD. Worked like a charm, lasted about 6 months. Then it started doing the same thing again.

With 195k on the clock, I figured maybe the calipers were hanging. Replaced the fronts, replaced pads and rotors, bled with computer. Worked great. Now it is doing it yet again.

I have bled this stupid car with a hydraulic bleeder, a suction bleeder, with the computer, and with the old fashioned method. It stays good for a week and then goes back to its shenanigans. I figured maybe the piece behind the master is bad, but you can’t get that bit without buying a booster from Toyota, which is about $850.

Does anyone have any suggestions?