If you are losing quantities you have a leak somewhere...easy way to check is look at the back side of the wheel if it looks wet it most likely a leak...check your lines, also check around where your master cylinder is..and or inside the cab....its a closed system so you shouldnt have any leaks...unless you changed your brakes recently you shouldnt have any fluid loss.
AUTO REPAIR HELP - THE MASTER CYLINDER

Master cylinders used with disc/drum combination brake systems will use a large reservoir for the disc brake portion of the system and a small reservoir for the drum brake portion.