I think you should forget about putting a vent hole somewhere in the unit. Where pressure can escape contaminates or moisture will also have an entry point neither of which should be in the system. I believe if a vent was required one would of been provided by the engineers who designed the system and unit. If you don't over fill the unit above the proper level (1/2" below the bottom of the fill plug hole) there is probably sufficient room for any expansion due to pressure and heat which might build up during use. Good luck but you should find your unit works correctly after being cleaned and new o-rings and seals installed, Hal.
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