I'm having problem with car starts and won't stay running and when pressing brakes it dies out.Works only when pressing gas petal.What could be the problem???? 1990 crown vic.

 

There could be other reasons for this problem, but I would suspect a good sized vacuum leak, possibly in or near the PCV hose. That might explain why the engine quits when you step on the brake pedal because the vacuum booster uses vacuum also and its enough of a vacuum leak to kill the engine.