DIAGNOSIS The rubber Safety Overpressure Plug has softened, swollen, or lost its shape from heat and use. It's designed to blow out if pressure gets dangerously high, but it should fit snugly in its hole in the lid when cold. If it falls through, it's no longer sealing.
FIX You need a replacement plug. They're inexpensive and crucial for safety. 1) Note the model number of your canner (01781). 2) Order a pack of genuine Presto replacement overpressure plugs from their website or a canning supply retailer. 3) When it arrives, make sure the canner is cool and depressurized. 4) Simply press the new plug into the hole from the underside of the lid until it seats firmly. The rounded head should be on the outside of the lid.
IF THAT DIDN'T WORK Double-check that you're installing it in the correct hole. It's the smaller, single hole in the lid, not the larger hole where the vent pipe/weight sits. Also, ensure the lid channel and the area around the plug hole is clean and dry so the rubber can grip.
HEADS UP Never operate the canner without this plug in place and in good condition. It's a critical safety backup. If your replacement plug also seems loose, the metal hole in the lid itself may be damaged or warped, and you should contact Presto customer service about the lid.