Brake pedal goes to floor with engine running

The engine is a 383 out of a car. I believe it has never been rebuilt. It seems pretty tired and doesn't have a lot of power. I had a vaccuum gauge hooked up to the carb when I was trying to adjust the idle mixture screws and it was pulling around 15. I pulled the hose off the brake booster when it was running and there was a lot of vaccuum there. That is one way I was told to check the booster for leaks. If the engine doesn't change idle when it is unhooked and go back to normal when it is hooked back up then the booster is bad.