What will cause my float bowls to go dry

I fill my float bowl manually. Once it becomes a routine, it can be done pretty quickly. Because someone was saying how they would dread doing it, I timed myself. 1.5 minutes from opening the hood to engine running with the air cleaner back on. Starts at the turn of the key, no long cranking.
The most important part is to find a funnel that fits the vent in your carb. Always keep it in the same place so you don't screw around for ten minutes trying to find it. I keep it on a shelf with a paper cup I use to pour the gas. I give the accel pump two squirts, and set the fast idle cam by hand on the next to highest setting. If I want to take 2.5 mins, I'll check oil, brake fluid, PS fluid and coolant.
If it's been several days or more since I've driven the car, I just do the procedure automatically.
Why not just add the fuel to the air horn, that way no spillage. I also have the same problem with my AVS on my 340 dart. I just rebuilt the carb and thought it would help, not. I'm kind of leaning toward the check valve because when I removed the fuel line from the carb, there was no spillage. As in plumbing (water) check valves do go bad. Never thought of that, Thanks.