Mystery Diesel problem

Here's the part that has me confused.. We had it running like that and after the level of fuel dropped in the bowl, we closed the fuel valve at the bottom of the tank and the level went back up!! The tractor kept running, too. We have no diesel in the crankcase. The fuel pressure gauge on the pump doesn't ever show much pressure but like I said, the tractor will sit there and run great for a long time. I'm not sure if it would do that with the fuel valve closed, but it did run for a little while that way. One time when we tried that, pressure built up in the bowl and caused the gasket to blow out of the assembly, squirting diesel all over the place. From the manual, I can't really see how this could happen. We have all new lines and filters installed and everything is super clean. It just doesn't make sense to me. Another thing I tried is removing the cap on the fuel tank but that didn't really seem to change much.

What do you guys think is going on here??? I'll be a better mechanic once I have this all figured out. Thanks for the help!

Good luck
JB2