If the slides or the brake piston its self are wearing out it can cause the brake to stick. Ive seen some pretty nasty calipers stick and wear down the pads very quickly. If the pedal isnt spongy i wouldnt think that it would have air in the lines. Doubt it would be the ABS as you will have a light come on when something is detected wrong. So Im leaning towards the clog or a sticky caliper.