Car stalls when stepping on brake pedal if I drive faster then 40 miles an hour.

Hello, depressing the brake pedal uses engine vacuum via the power brake booster to provide power assist to make the brakes engage more easily. It's possible that an engine vacuum concern, or air intake concern could be causing the engine to stall when using the engine vacuum for the braking system. Too hard to say -- so hands on testing is needed to figure out what's going on. A general engine performance issue alone can cause a stall when engine speeds diminish anyway - so testing is the way to go. Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops