Surging standpipe?

If it is just leaking from the overflow box at the bottom thru the bulkhead down the overflow then I doubt there is any air getting in from there. Are the height of the pipes fixed or adjustable? Does making one a little higher than the other change the problem at all?
 
I would still open up the hole and put a valve in the overflow that's giving you issues. Just because everything seems identical doesn't mean you're not going to have issues. Water moves in a unique way down each pipe and apparently is causing some discrepancy.

Adjust the airflow with the valve on the top of the durso to see if that helps solve your issue.
 
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