I had the front (disc) brakes on my Beetle sieze on while on the way to Strouds anniversary trial last year. At the time I changed the calipers and finally got round to looking at the old ones last week.
The problem ones weren't that old, around 4 years. When I took them apart I saw that the cylinders were corroded outboard of the sealing ring. The bores inboard of the seal (where the brake fluid sits) and the pistons were fine.
The protection seal was fine, no holes and seated properly. I believe that water got in when using a pressure washer to clean down. I am a bit religious with this and jack the car up and take the wheels off to clean underneath. No more pressure washing for me!
Michael