izanami

I just replaced some coolant hoses and was trying to pressure test the system with an autozone rental tester, but I couldn't get the cap to seal on my pettit AST. It is strange because I have used the testers on the car before and they worked fine. The tester will seal on the pressure cap coupling, just not my AST. I am kind of curious if anyone else has had this problem, but I am more worried that my pressure cap may not be sealing. Is their another easy way pressurize the system with the pressure cap on, so i can make sure it holds any pressure?

Cheers,
Dale