I had the same problem. I stole the prop valve from my Scamp, rebuilt it using the guts from one of the new, leaky valves, and put it in my wagon. That cured the leak.
I e-mailed Inline Tube about it and they said they never had a problem like I experienced. I asked them what fitting they used in the top port if they never had any leaks, but they didn't respond to that question.