i have a 97 gmc sierra and whenever i press on the brakes now it resists it and i have to push really hard to even get it to stop. when i put a used rear end in one of the bleeders wouldnt come loose to bleed all the brakes. could it be air in the system or something else. i've had the used rear end in since early 2010 and no brake problems til a couple days ago. it makes the sound when u first press your brakes and it doesnt stop making it unless u push extremly hard and get it to the floor. if anyone has any help it would be appreciated