[Fulltiming]

The cap on the radiator itself is the one that controls the pressure the radiator is seeing. The cap on the overflow tank should be the same pressure. If the overflow tank cap were higher pressure, when it reached its pressure the cap on the radiator could not open and overcome the additional pressure. If the cap on the overflow tank were lower, the overflow tank would bleed off first. If they were to be different pressures, I would err on the side of making the overflow/fill tank a lower pressure but it is better to have them both the same.