I just don't get it..

i chased around for a cap that would work in my original radiator 68 RR, tried about 4 different brands would only have some PSI when it got very hot and that was hit or miss even had the later style 72 mopar one on there, Do yourself a big favor and try the repo ones they sell, as soon as i tried it, it held PSI just fine even after about 5 min run time way better than the part store ones.