Whenever I try an open string pull-off I either make way too much noise or not enough, what's the correct way to do this technique? Should you lift your finger directly off the string quickly or 'pull' the string like you do for regular pull-offs?

it's like a standard pull off. the key is to have enough left hand control to not make noise on the other strings. tying can help, and so can muting with your right hand
That's because an open string is looser then a fretted one - the finger fretting acts to control how far the sting may vibrate to some extent. Practice it slowly and cleanly, first on a purely clean setting, then add gallons of distortion to make sure you can play it at both extremes. Or try it on an acoustic - those really force you to have good technique.