Try loosening the flare nuts upstream of the flex lines in the front. It's unusual for both flex lines to fail at once, but not impossible. If the calipers stay applied with the metal lines vented upstream of the flex lines the flex lines have collapsed inside.

Edit: some boosters have an adjusting screw between the booster diaphragm and the master cylinder, but that one would probably affect all the brakes if it's out of adjustment.
Jerry


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