deffo sling a 1way valve in it before the atmo breathing bit mate, facing outwards.

on S13s, if you dont run one, they do a fantastic impression of a totally fucked and mega smoking engine, but are fine with one.

may be the same prob with you (as as standard it will always have the inlet sucking on it, so contant vacumn, so need for a 1way), so it DEFFO worth a try mate!