67 Dart cooling issues

""This thing doesn't push water out into the overflow does it? Nope""
It should transfer water in and out of the coolant resivoir. That's what the resivoir is for.
Like a 16 lb cap should release pressure above 16 lbs.
Is it possible that just any brand cap doesn't fit and operate properly on these aftermarket radiators ? If you stand on the cap with both feet to install it, it aint right.