hydroboost question

[magnethead]

ok, i'll say off the bat that my rear drums are 20 years old. Front discs are about 5 years old.

on my truck, whenever i press the brake pedal, it tends to change in firmness. I've gone through 2 MC's in as many years, and they really only last about 6 months before the pedal goes back soft. but now i've discovere that if i left foot brake and pop the throttle while braking, the pedal becomes hard as a rock briefly (only about 2 seconds). A few times i've done it i've had the pedal halfway down, and the second i pop the throttle (without pushing my left foot any more than it was) the right front will lock up.

Does anybody know what might be going on? I'm wondering if the pump isn't making enough pressure to run the brakes, or if the hydroboost unit between the wall and MC might be going bad? I've also replaced 2 hydraulic lines, one was punctured and the other was worn. Also, if i turn rather sharp, the pump starts squaking pretty bad. It's unavoidable, it mostly happens when i go to school, i have to be on the other (read: wrong side for right handers) side of the road from the turn to be able to turn without it squaking. Fluid canister is full.