Fixed ABS light, now the brakes grind?

[tx_rooney]

I have a 2000 Explorer Limited AWD. The ABS light has been on since I bought it in Jan. The front rotors and pads were replace in Feb, but I just now changed the rear differential sensor to clear the ABS light. Since then, the brakes grind on every stop, feels like it is pushing on the pedal. The master cylinder looks like it has leaked some in the past where it mates to the booster. 145,000 miles.

Is this actually the ABS pump pushing back on the pedal? I'm tempted to replace the master cylinder but value your opinions if you've seen this before.

Thanks!