You could have a cracked tank,like I did...

I just bought a new tank and rebuilt the radiator after it was rodded out.

The crack in mine was on the left side tank about 3" above the outlet tube and it only leaked after shutdown (until it blew out)

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"When the Team Chief said.... You're trapped in a hole with nothing but a goat and a slinky, what do you do? Stubby said, I'm not sure but it won't end well for the goat...." ~(Overheard) Last day, Phase 3, Q Course