i think most new stuff use a 16lb cap. and by new, i'm talking about the OEM 4-ear cap that came out in ???? model year.
make sure the cap you are using has the two gaskets, and the inner one fits and seals good on the bottom of the radiator neck.
also, be sure the overflow hose is tight on the radiator neck, and the catch can has a good vent to atmosphere.
sometimes it takes several cycles to expel all the air from the system, so don't let the overflow run dry.
you may even have to get the nose up in the air to coax the air in the system to rise to the top.
i squeeze the upper hose while filling. that seems to help burp it some.