I would first suspect a rubber hose failure on the inside. If you think they may be original, they are overdue for replacement. They are known to fail in the other direction also and prevent fluid from returning, thereby keeping the brakes applied.

BTW, the old fashion way off pumping up pressure with the pedal can cause problems. The inside pressure cup is forced to travel outside its normal range and encounter less than smooth surfaces.

Harry
86 300 SDL
85 300D Euro