Phil Graves , According to your manual this may be caused by mineral build up. A common problem with steam appliances is mineral build-up caused by high mineral content in many municipal water supplies. The best way to avoid this is to use de-mineralized or distilled water in unit. If you are unable to use de-mineralized or distilled water, it is recommended you use a mixture of 8 oz. (237 ml.) vinegar and 8 oz. (237 ml) water to help remove deposits poured into the empty reservoir, let sit an hour. May have to repeat process if build up bad. Use the link below/manual for further instruction. Good luck. I hope this helped you out, if so let me know by pressing the helpful button.

http://www.mccullochsteam.com/Manuals/MC...