Did a leak check last night, after I started the car I left the oil cap off (by accident) and noticed a lot of pressure excaping from the engine. Put the cap on and then take it off and PPPSSSSTTT! A ton of pressure. I thought the crankcase was supposed to be under slight vacuum? The pancake valve blows through either direction so it can't be holding back pressure. This can't be good for gaskets and seals. What am I missing?? '98 A4TQM.