I had a similar issue and discovered that the nipple was compromised on my over flow tank. I bought a brass nipple, slightly bigger than the stock and drilled mine out and jammed the larger two sided brass nipple into the overflow tank until I could get someone to deliver me a new tank ($50). That overflow tank seems to be pressurized so a cracked nipple allowed for seepage even with 2 clamps on it.

Check that nipple and see if it's cracked.