I would definitely not use any pressure beyond what a vacuum cleaner can generate. Besides the hulls are vented in the pylons and there should never be any pressure build up. If you pressurize with a vacuum cleaner like a shop vac in blow mode you should have enough volume of air flow to generate a pressure enough to find bubbles in your soap solution. Whatever you do, don't connect any compressed air to that valve you installed in a plug. Assuming a hull is 16' long and averages 1.5' high, a 1 psi pressure would cause a force trying to pull the hull apart of over 26,000 lbs. A vacuum cleaner generates pressure in inches of water, maybe 3 or 4" where 1 psi is equal to over 2' of water column.