2006 Lexus LS430–Brakes–firm pedal sometimes, but pedal can be mushy. Pedal can be pushed to the floor engine running or not. When creeping up to a barrier, brake pedal is soft and loses stopping power. Replaced every thing in the braking system except two rotors that are good. Dealers and Lexus Corporate says "take to dealer’, There must be something is the system that fails intermittently.
Have the car towed to a competent independent mechanic, do not drive it as the brakes could fail at any time. A dealer is not necessary but someone who knows what they are doing is. This sounds like a master cylinder with an internal leak. What other parts were replaced?
Yeah, sounds like a MC to me as well. You sure that was replaced? If so, it is possible though fairly rare to get a bad MC out of the box.
I find that statement a little strange . How many places is this person talking to instead of just finding a place to fix the brakes . Good brakes on a vehicle is not an option or something to waste time repairing.