Pressure testing a radiator cap?

[Dozerboy]

So I pressure-tested my radiator cap on my truck and it was rapidly leaking. I couldn't build but a 10 PSI pumping like hell with the test kit. So I ordered a new radiator cap it does the same thing. Is my new cab bad or an I wrong that my cap should almost hold 15psi and slowly leak down?

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[Dozerboy]

Second new cap still the same out come. I'm guessing it's the tester some how. The screwy part is I tested 2 caps on gasser and they hold pressure like they should. The caps have the same part number as mine too.

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