if he has a leak im sure he would know about it very quickly. seeing as every time he hits the brakes fluid would come out due to the large amounts of pressure. i know on some cars the pedal drops a little normally no matter what you do. but usually you dont use a USED master cylinder. get a new one or one that you know works just fine, as in a running car and try that if that doesnt work then you might have a issue with your brake booster.