The cork has to be adjusted from time to time; I'm guessing the new ones from Moss probably need to be adjusted right out of the box. Loosen the lock nut on top of the handle then use the screwdriver slot to turn the shaft CCW until the cork will just barely move in the bore.
I got tired of having to adjust mine many years ago, and found a piece of nitrile fuel line that fit. Seems like it was 5/16" id, but I honestly don't remember for certain. Anyway, I think I only adjusted it once in 20 years or so. Of course, I don't use the tap very often either. Since my driveway slopes, I can just park the car with the nose up to clean the sediment bowl.
Steve, I'm actually surprised your car has the fuel tap. I thought they were deleted around TS60,000 ?