I found alot of other people with this pump crying about it too, so it appears it doesn't have a check valve. It just has me a little scared that there's potentially something else wrong I'm overlooking. I should be hitting the dyno soon to finish the tune, so I'll know soon enough.
I guess that's not a big surprise, given their universal nature, I have always used Walbro pumps and they had check valves... but I never ran the High Pressure 255 model before.