Thanks guys!
DMGfifty - Its a brand new cap. ...but I'm thinking about replacing it with a locking one anyways.
Cletis - That's what I thought, the cap vent only working one way. Didn't realize the pressure could build up that much tho.
80_Grit - thats not a bad idea. I have an extra filler neck in the parts bin. I might solder on a nipple and attach a line.
