What I did:
I have a 1997 GMC Jimmy that I replaced the power brake booster and rebuilt the rear drum brakes (new brake shoes, springs, and wheel cylinders). I bled the rear drums since I replaced the wheel cylinders. I did not have to remove any lines from the master cylinder in order to remove the power brake booster / install the new one so I did not bleed the front brakes. I also topped off the brake fluid in the master cylinder.
When I took it for a test drive, every once in a while the brake pedal would stick to where I pushed it to and there would be a squealing kind of noise coming from the area of the brake booster. I took the car on about 5 test drives and every time it would do it at least once. Sometimes it would happen when I slam on the brakes and sometimes it wouldn't. Sometimes it would happen when I was just pressing regularly and again, other times it wouldn't. The easiest way I found to get the pedal to come back up to normal position was to put the car in park and pump the pedal.
What could be causing this? Is the power brake booster that I was sent defective or is it something else? Any suggestions or ideas would be greatly appreciated!