I have an American Standard showerhead (brass) in a stall only no tub that leaks. I have replaced the rubber washers in the showerhead and it still leaks. Could this be caused by the one handle for both hot and cold ?

Does it leak all the time or just when you are showering? It must be the control valve if it leaks all the time. If just when you are showering try some teflon tape at the head connection.