Craftsman, Walbro 424C venting issue

My Craftsman leaf blower starts OK, runs for a minute or two and then slowly dies like it is starving for gas.  When I push the priming bulb, a small amount of gas sucks out of the tank into the bulb, but then the bulb gets hard and does not push gas into the carb.  I removed the carb, pushed the bulb and the small amount of gas that was in the bulb, did not push out.  I rebuilt the carb this winter, but I am having the same issue before and after the rebuild.  I cleaned the metal parts in Berryman's carb cleaner and blew compressed air through the passages. What passage(s) would be blocked that would prevent gas flow?  I didn't replace the welch plug because I wasn't sure if I would install it properly, but I don't know if that would cause this issue.
Any thoughts?  Much appreciated.

Yeah, AVB is dead on right. I seen one just like this caused by just using compressed air. Purge bulb will get really hard, people like to call them primers but most are purge bulbs.

At least it was one those cheap cubes instead of one those $100 cubes.
 
I don't know why Walbro even sells spray carb cleaner. Generally if I know the user have tried to clean a carburetor on their own I generally know I am in trouble. Normally I have close to 95% successfully rebuilds but there will always those that can't be rebuilt. I got a C1Q-W11 that the metering needle seat is leaking around it mounting area so it is getting a new carburetor of those that blower carb have been reworked several times over the years.