Jetcopterpilot, thanks for that link. I have never seen this site before and I think that I am going to try that cap. Oddly enough, the cap that I have on my truck now, that was sold to me as the correct one for my truck, looks exactly like the one pictured for the 71-72 trucks on your link. Might explain why it doesn't work!
Thanks to all. I might buy a couple of other parts from them too when I get the cap.

Thanks again, Patrick