well the tank breather should have a check valve to only allow ambient air into the tank (without a check valve the breather will leak in a rollover...bad). To build up pressure in the tank (from thermal expansion and such) is usually the cap itself (kind of like a coolant pressure cap). If you are building up pressure, you have the wrong cap. you are talking positive pressure, and not vacuum pressure, correct?
ISIL and al qaeda can go F*ck themselves...