One problem the crud/water ingress seems to cause is clogging up a metal-to-metal connection that allows the red pressure warning light to stay off when there is only light pressure, or no pressure at all on the toothbrush head. When the LED is stuck ON the toothbrush works fine, but it's annoying and will drain the battery quicker than usual.

The metal surfaces you need to clean up if you have this problem are about 2cm down from the external power button. Lift the black plastic channel (careful - don't damage the metal strip it's enclosing) and clean between this strip's T shaped end and the metal it should be making clean contact with underneath.

You can try alcohol and a Q-tip first or if it's really cruddy abrade with a knife blade, emery board, sandpaper or similar. Take your time and be gentle!