what is it and what does it do? 89 xj w/ abs

http://www.youtube.com/watch?v=mB8MZScV6VA

it only goes on when you touch the brakes and when i first turn on my truck. its connected to the master cylinder. the noise it makes has changed and my brakes have gone really soft. they turned soft before the noise changed. i cant find any info in my FSM. does some one have any info on it? is it supposed to make noise at all? whats it do?

That is the infamous power booster pump. It runs every time the pressure in the accumulater drops; like when you push on the brake pedal. Mine makes a humming noise, but it's nothing like that. Is the brake fluid in the reservoir topped up?

FWIW, When you start the engine, the ABS solonoids cycle IF the accumulator has adequate pressure. When I start the engine, I lightly hold my foot on the brake pedal. If I don't feel the "thock! thock! thock!" I know the pump may not be working.

i dont have any resistance on the brake pedal when i turn on the car. would the pump be bad or master cylinder