Hi Ronald,
Congrats on acquiring your Mac19–it’s an awesome little boat and they’re hard to come by.
Most likely your 2” Valterra ballast fill gate valve on the transom is leaking. You can order a new one online—it is a common and inexpensive RV part and easy to replace. You should also check out your centerboard to make sure there is no damage to the trunk and that the swing bolt is sealed. You can access the interior of the ballast tank by unscrewing the two 6” inspection plates from the cabin sole. Make sure you properly clean and reseal the plates when you re-install (if they leak the cabin will flood when the ballast tank is filled). I resealed my centerboard swing bolt and inspection plates with LifeCalk.
I use my Mac19 almost exclusively as a power boat, so I removed the gate valve from the transom and replaced it with a 2” stainless steel/neoprene expansion plug which keeps the ballast tank water-tight (filled or empty) but is a pain-in-the-rear to remove/install when on board. Install a new expansion plug in the ballast tank air vent under the cabin step while you’re at it to prevent the cabin from flooding.
I do not recommend using air pressure to check for ballast tank leaks—the tank is not designed to handle that type of load and you could actually cause a leak if you blow out a weak tank-to-hull seam somewhere. And because ~90% of the ballast tank is inaccessible (without tearing out the inner hull liner) there’s a good chance you couldn’t find or repair a ballast tank leak anyway. Furthermore, unless you have damage to your hull or centerboard trunk, a leak in the ballast tank joint would only cause the hull to slowly flood when the ballast tank is filled.
Hope that helps & happy to answer any other questions you might have!