Why would a brake booster not return after releasing the brakes even with the new master cyl removed?

there's a diaphragm inside the booster that can get stuck and not allow the rod to return all the way,if that's the case you may have to replace the booster,most of the time when they do this the booster is bad on them,even if you get ti unstuck the diaphragm may still be bad in it,good luck.