When the pedal returns fuly it uncovers the holes in the m/cylinder, which then allows it to draw fluid in to it's bore and be pushed by piston action upon pressing pedal again.
Even if it had a totally jammed caliper piston it could therefore not create pressure once pedal released.
It could well be that the piston in the claiper is sticking and causing a drag but this would not create pressure in the caliper when pedal released, as hole uncovers and any pressure is released back into m/cylinder reservoir.
tabetha