Usually the check is a ball in an external fitting on the poly tanks. Looking at the steel tank pictured, if it has one, it would be on the inside of the tank. Don't know if you've got visibility to that area to inspect. Also, is it possible you have one of those "sock" type filters on the end of the pickup tube? It could be it's clogged with debris that when you blow through it it acts like a "flap" in that it would allow air to push the material away, but when suction occurs it pulls the debris tight, blocking flow.