When was the last time you flushed/changed the brake fluid?
If it's been more than 2-3 yrs., the fluid could be so saturated with water that it's boiling and not fully transmitting your pedal action to the caliper pistons.
Also, can't rule out the Master Cylinder either.
Minimum pad wear s/b 2mm (0.08"). You get two sets of brake pads before requiring a rotor change. OEM pads cost me $120 for both axles + $22 for new brake fluid.
