There is going to be some amount of air coming out of where the comp tip and the real mast
meet no matter what. there is going to be some gap between the plug in the mast and the plug in the comptip.. and it is going to have to be under for some time before all the bubbles attached to the luff track will clear themselves out.
From what I have seen taking these things apart I would be surprised if one of those plugs were leaking (though not impossible) It is far more likely a rivet in the mast tang or wherever is leaking..
If it is really leaking at the plug below the comptip I would think the only 'proper' way to seal it would be to remove the comptip and then fix/re silcone the plug. (which is doable.. but not fun) I don't think there would be any way to really properly seal it up without doing just that. Just gooping on something between the tip and the mast is not likely to hold up IMO...y
Can you use a shop vac to apply some pressure to the mast where you drained it from and soap and bubble check it?