Michigan State University Extension
Preserving Food Safely - 01600735
Sometimes, a lid on the pressure canner will become "stuck" if the lid is not removed when the pressure has reached zero. NEVER try to hurry the cooling of any canner or force it open by pouring cold water over it. This shock treatment could cause the seals to loosen, the liquid to siphon off, or the jars to break. Reheating the canner and letting it return once again to zero pressure should release the vacuum.
Other methods that are recommended:
Using a rubber mallet, gently tap around the rim. Then, exert equal downward pressure when turning the cover. Never use a regular hammer. If the canner lid is stubborn due to a sticky sealing ring, it's time to replace the gasket. Rings are prelubricated before packaging and only need oiling occasionally. Oiling the gasket too frequently can cause swelling, rapid deterioration, and even more problems.