[davepl]

I don't want to guess whether you should have a vented cap or not - since it's a C20 there will be no charcoal cannister, but it sounds like you have pressure in your tank, and hence you should have a vented cap. But that's just a guess, find out for sure for YOUR truck.

You say you removed "EGR and pollution stuff". Some of that "stuff" can be important, it's not all junk. Your carb is calibrated for having EGR in the mix and it does NOT hurt perf, so.... your call on that of course.

That pressure won't hold the pump open, but it will "supercharge" your fuel system and force fuel in at a pressure that is (a) higher than expected, and potentially (b) higher than the needle and seat can stop. That is what I expect is the root of your problem - it's pushing fuel past the "valve" at the front of the carb.