my 1990 cadillac runs good until you hit the breaks than runs rough

Hello, it sounds like a couple things could be happening. You have a vacuum operated power brake boost system. When you are dealing with engine vacuum, a concern with your engine performance or with the booster can affect the way your engine runs. Secondly, anything that puts a load on the engine can make the idle speed lower, and in turn can emphasize some problems with the way the engine runs that you don't notice with a higher engine speed. In any case, testing hands on is needed; i'd start by looking at all vacuum lines and connections.