As Brian Carlton stated earlier, get a pressure device and put the system under pressure. The other quick thing to check is your veggie fuel filter. Unscrew the bottom drain and see if anything other then veggie oil comes out. (warning... by doing so you'll introduce air in the system so make sure you hand pump the air it out afterwards) If you see something different it could be one of the internal o-rings or veggie line is compromised.

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- GreaseCar Veg System