could be weak rubber lines going to the caliper and swelling when brakes are applied feeling spongy and one could be fine and not the other letting it grab sooner,air in system,also sticking caliper will make more fluid disperse to opposite side quicker if its slow to grab causing distribution surge and if you barely lay foot on pedal and it keeps goin down till theres no pedal it may still be the master cylinder and if its air in line the one side thats not grabing is most likely where the air is...