You may be on to something. I have found several instances in customer cars where a stuck piston on one side of the rear Ate calipers causes the good piston to push the pad over and distort the rotor enough to make it contact the fixed pad. Over time the pad wear causes a huge gap which is taken up by master cylinder travel, making the brakes feel spongy, even though there is no air in the system. Pull the back wheels and confirm near zero clearance on the pads on both sides. Have some depress the brake pedal (engine running) HARD and observe the rotor edge.