contaminated brake fluid

[Robert Squires]

* Every now and then I see a car with brake hydraulic problems which stem from something or other that has swollen the rubber components in the master cylinder. The fluid surface in the reservoir looks like the surface of a vinaigrette salad dressing, with the oil globules floating on the surface, and there's usually plenty of sludge on the floor of the reservoir.
* Has someone mixed silicone-based DOT5 with the DOT4 to get this mess? Or is it an age issue or a moisture issue?
* I normally use ATE SL DOT4 fluid in all my brake work, and as far as I know DOT3 and DOT4 are compatible (if they happen to end up in the same reservoir).