The cap has a little one-way valve, its probably gummed up. Soak it in some wd40. As for priming, the walbro carbs use a circulation style priming system. There should be two fuel lines that feed the carb. One is probably green, and one is probably clear (or orange by now). When you press the primer bulb, you are drawing fuel up through one line, cycling it through the carb, then back down into the tank through the other. You should be able to prime it with the tank completely sealed. The check valve in the carb might be stuck. If you take off the primer bubble (4 screws), you should see a little orange rubber thing. GENTLY move your finger around on it. If it feels like its loose and can be moved around, its working properly. If it feels like its stuck, then that is probably the cause. If it is stuck, you can attempt to un-stick it by gently pulling it up. You have a 50/50 chance of damaging it, and will have to order a new sub body for the carb if you do.