You guys were absolutely right! Today I made some fittings to pressure test my cooling system with the pressure cap installed. And surprisingly it did not hold pressure, and I mean it did hold absolutely zero pressure! I purchased the expansion tank cap at a well known dealer - who by the way sold me the completely wrong t-stat as well! You got to be kidding me - I buy at a specialized Pantera dealer and get 2 products that are not correct. Very frustrating!
And yes, the tank is o.k - I mounted the old cap and this one holds pressure as it should be!
Thanks for all the help!