Can I damage my GSD's coat by brushing him to much?

My German shepherds coat seems to be dull and brittle right now. I'm always brushing him, Trying to keep him looking nice. I've noticed his coat seems to dry and brittle. I bought this hair conditioning cream called CHOLESTEROL. It's made by Queen Helene- professional formula. This seems like some pretty good stuff, My problem is I'm sure on how to use it. Can anyone advise me on how to use this hair conditioner.