Are either of you using brake dust shields? Several years ago I was using the dust shields in my 92 400E and the brakes failed much like you described. They started working again in a few minutes after things cooled down a bit. It was a hot day and I had just been driving pretty agressively to rush across town in heavy stop and go traffic to get to a store before they closed.

My shop pulled off the dust shields and changed the fluid. Brake fluid absorbs moisture and the brake dust shields reduce the brakes ability to cool. That combination caused my brake fluid to boil inside the calipers. I've since had many many happy miles with no further strange brake problems.

I won't use the dust shields and I get my brake fluid changed every couple of years now...

Good luck!

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02 S500
92 500SL
92 400E (Sold)
87 300E (Sold)
83 300D Turbo (Sold)
75 300D (Sold)
74 240D (Sold after 20 years)