In that case i would suspect the piston cant go further in because there is some rust and muck behind it. I would take the piston out (using the brake pedal to move it out - minus the brake pads) and clean out the bore behind the piston.
Or could it be that your caliper is mounted on pins so it can slide sideways to take up for the outer pad wear - and the sliding mechanism is stiff. I had the problem on the front of my 92 LX.
Or could it be that your caliper is mounted on pins so it can slide sideways to take up for the outer pad wear - and the sliding mechanism is stiff. I had the problem on the front of my 92 LX.