I need help with brake question. I'm having problem with rear brakes sticking on my car.(everything is new) except poportioning valve and hold off block?(I think thats close to its name) but anyway I can break loose either bleeder on whl cyl and you can hear both sides let loose and as soon as you hit the brakes again pow locked up again, Have vacuumed bled and still locking up. What does the hold off block do? Please help......