I had the same problem with my '87 wagon when I acquired it. I had a stuck rear passenger side caliper and I replaced it with a used one (only the one side). I repeatedly bled all four calipers with 4 quarts of brake fluid and couldn't get the light to go out. I could press really hard and it would flicker out, but as soon as I braked normally, the light came back on. I drove the car for a month, re-bled the two front wheels, and the light went out (and stayed out). I'm guessing that I had a stubborn air bubble somewhere in the system and the vibrations of driving got it to move to a point where it came out. A tiny air bubble would cause a failure light at light duty braking, but would compress to almost nothing when heavy force is applied (thus balancing the pressure under heavy braking and cause the light to go out momentarily).

I'd wait a while and try bleeding again. God bless,
Fitz Fitzgerald.
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'87 Blue 245, NA 220K