You may have other issues along with the wrong radiator cap. If combustion gasses are getting into the cooling system it will purge coolant well before the system reaches operating temperature. Or it could be as simple as an air lock-not purging all of the air from the system as you are filling it.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy