Depending on the pressure rating of the cap, it might be doing what it should. I have a new radiator cap at the correct spec of 7 psi, I added an overflow bottle to the system, it keeps the radiator 100% full and the bottle takes care of expansion and contraction.
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
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