Hi Alex V.
If possible, see if you can find a fiber gasket (or make one if you have the facilities) that will fit in the same place as the O'ring.
How many "bubbles" were coming out....just a few,, right away, or did it take awhile and 'only a very few smal bubbles came out?
It sounnds like it's leaking for sure, but air being pushed out from the inside might be almost normal with the O-ring setup used on your board.
It may be impossible to get a full pressure tight seal.
For now, I would get so non-hardening gasket sealer (Permatex makes one) and put a little of the gasket sealer (you could use bathtub caulk,RTV rubber, or polysulphide based sealer as well) on the O-ring and see if that doesn't stop the internal pressure leaks and still allow you to remove the vent plug.
Use only a tiny bit and make usre it's something soft and rubbery that will break loose when you want the plug out again.
And, as I suggested before, a small square of duct tape over the vent plug hole as a back up.
Hope this helps,