Which type have you got? are they the old blue type, or the newer type shown in the pic?
We've had both, and the old blue ones were more bother but it was the inside of where the milk goes through that gave the trouble. When they came to it, the dealer said it was the poor quality chemical we were using to clean it. The best one we found was the Alfa Laval powder and did a milk stone remover wash more often.
We used to prssure wash them 4 times a year but didn't have that much trouble inside.
The newer ones we have in the 20/40 (see photo) have been very little trouble at all and we pressure wash them bi-monthly. I know we shouldn't, but we do. We've been told not to but it's the best way to clean them. The touch pad is wiped clean with a TFR mix, soaked cloth once a week.
http://img.photobucket.com/albums/v371/JamesF/Fullwood20-40.jpg