For a coffee brewing machine, an indicating system which operates to notify the user when it is time to clean the water supply conduit of accumulated mineral deposits. When a self contained removable reservoir is inserted into its operative position connecting to the water supply conduit, it closes the contacts of a temperature sensitive switch. During normal operation, the switch opens when water is depleted from the reservoir and the brewing cycle is completed. However, if the switch opens while water remains in the supply conduit upstream of the heater mechanism, this is a sure indication that there is a substantial accumulation of mineral deposits in the supply conduit. Thus, a sensing mechanism is employed to detect the presence of water in the supply conduit after the temperature sensitive switch opens, and a lamp is energized to indicate the condition. In one embodiment, the sensing operation is performed by an electromechanical device, and in another embodiment by logic circuitry.