I just fitted a new OE radiator out of a Pontiac G8 to my car and run a factory 20psi cap. I went to open the cap the other morning when it was stone cold and it had pressure and spat a bit of coolant out. There shouldn't be any issue if it holds a small amount of pressure, your cap is doing its job.
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'71 Holden HQ Monaro - 3850lbs race weight, 400c/i - 11.4 @ 120
'66 Pontiac GTO - 389, 4 speed street cruiser
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