Have you looked down in the radiator to see if bubbles are coming up inside?
First I would test the cap. New doesn't always mean good.
Then I would put a pressure tester on the radiator to see how fast it's building pressure. If it's not hot but building enough pressure to blow past a 4# cap there might be more going on than just the radiator.
“The rat rod is a whole lot of fun, in theory, until someone forces you to actually drive it.” - David Freiburger, Hot Rod Magazine
Kentucky Chapter Member