90 SC bird brake issue

A friend of mine is trying to sell me his 90 Supercoupe. Only thing holding me back is a strange brake problem it developed recently. It stops fine when cold, but after about ten minutes of driving it seems to loose the power assist for the brakes, makes for a scary ride stopping a 4000lb car.

This car has the ABS integrated booster unit, which I hear is not exactly cheap to replace.

Anyone have any ideas.....
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