Figuring out what is wrong may not be easy. You might try to start with the basic brake stuff like doing a full fluid system bleed, master cylinder test, etc. But once you go beyond the basics of the brake system, it may get very complicated. If the Brake assist is coming on too easily, there must be some sensor that determines when to activate it, and maybe that is the issue. You have officially exceeded my knowledge base. Good luck, but do let us know if you figure it out!