I agree with the "boiled fluid" theory. Brake fluid that has picked up a lot of water will boil at lower temps, and if one caliper is sticking, you can boil fluid in that caliper blowing the fluid back into the master cylinder. You stopped because you still had the rear brakes, assuming it was a front caliper sticking. See if one wheel center is hotter than the others. If so, that is your problem caliper. Fix it, and use fresh high boiling point Dot 4 fluid.

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'97 E 300 D