Powermaster motor is on the way out or the brake pressure switch is bad(not cutting off at correct pressure) or both bad. I assume you have the proper brake fluid level and not running dry. All will cause a high pitch squeal.
advanced auto parts (among others) sells a complete system for $250. that's the motor, switch, lid, the ENTIRE setup. it's also got a lifetime warrenty.
i bought one of those a few years ago, now i'm done spending money on my brakes.