I have had similar results from TOTALLY gone pads on the rear calipers.
I'd TOTALLY recommend a new MC, but be certain the pads are thick enough to allow the MC compression.
My 87 wagon had "pedal to the floor before it'd grab syndrome". replaced the MC, no change. TOTALLY FLUSHED entire brake system... no change... replaced the rear pads and calipers, and POW! full solid pedal again.
no fluid was leaking.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!