Tom, something I read or heard a while back was that the older styles of brake fluid absorb moisture out of the air over time. This would make its way to the lowest point in the system, being the wheel cylinders.
While the reservoir has what appears to be a rubber seal under the metal top, there is a tiny hole to allow air in and out. The idea is that when the brake is pressed and the master cylinder piston moves, the fluid level drops. The air has to get in and out as the fluid levels change. Any air coming in carries tiny amounts of moisture and over time they would build up.
If you are looking for a long-term solution, do what Restless Rover (ex Red Rocket) did, and have the cylinders sleeved with stainless and get someone with a lathe to turn up a couple of stainless pistons. Its a simple and quick job and shouldn't cost much. I intend doing it to mine shortly.
Restless Rover used 3 of his MK3's for years as part of his business and said that once he sleeved his cylinders, he never had another issue with them.