This is just silly, but hey, it happened to me...
And no one else offered any ideas...

The reservoir is divided into 2 chambers, front chamber for the front brakes, rear chamber for the rear brakes. I found that the rear chamber only gets filled when the front chamber overflows into it, which surprisingly happens just at or perhaps a bit above the 'full' line.

I was running the rear chamber dry, and it took me some head scratchin to figure this out, since the front read full. If you clean up the outside of the reservoir, you can see where the rear chamber level is at.

Your reservoir may differ from mine, since you have a V8, and I have a manual (with clutch supply).

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1986 300E 5-Speed 240k mi.