3D Student said:
I've got a 6 quart Presto pressure cooker that's 4.5 years old now. I have used it about once a week so it's been through hundreds of uses. Just make sure you buy extra rubber gasket rings if yours uses them. They seem to wear out in 6-12 months. They will shrink and the pressure cooker is no longer airtight and leaks air from the sides.
I agree with 3D Student on the rubber gasket rings however if you purchase a pressure cooker from a garage sale or flea market you can revitalize the gasket. Your used pressure cooker/canner might frustratingly fail to seal on the 1st, 2nd, and 3rd try

This seal is the most important thing to check before your purchase. Upon inspection if the rubber ring looks dried out or cracked... forget it. If the asking price reflects the condition of the cooker and your competent in knowing where to order parts for your great find , then great. If the rubber seal looks satisfactory it can still be a crap shoot... so what to do if it doesn't seal.
PRESTO CANNER:
Remove the rubber seal and hand wash in warm soapy water. You will notice the seal is actually a U - shaped gasket that's in need of manipulation to bring about it's original form. Hand manipulate the U - channel by running your thumb through the channel applying pressure to the bottom in order to spread the wings back to their original shape. This will take more than just a little time and effort... but hey, this was a bargain so make it work. Move around the circle. Repeat the same procedure several times until you see that your efforts are paying off.
This might be all that's needed for the seal to now hold the pressure as intended. After doing the above and still no seal do the following: Find a small piece of rope that will fit into the U - channel albeit grudgingly. While seal is still warm and flexible force the rope into the channel all the way around the seal circle then cut the rope to fit. Try to force the lid of the canner on with the rope remaining ensconced in the seal. This can take some effort and if you don't succeed in closing down the lid you are still accomplishing a stretching of the seal back to it's original shape (somewhat). Hey, remember this was a bargain.
If you think you have more than accomplished the above then go ahead and try and see if the rubber is stretched significantly to provide a seal.
If your cannier still doesn't seal, put the rope back into the seal channel and leave there a couple of days to furthe stretch the seal. This is the way I store my canner when not in use so the rubber retains it's original shape. I have heard that some have used olive oil on their seals when storing away and also when in use.
PRESTO COOKER:
Some Presto cookers have a flat seal that stands on end in a small groove. Remove, check and clean as above and clean out the lid groove. Lightly coat seal with olive oil then reinsert.
For me not having the cooker/canner seal after bringing up to temperature is par for the course, get use to it, although frustrating. A dry run before your jars are packed or your food is in the cooker/canner might provide a "heads up", ;)
I really like the feeling I have when I'm canning. Good luck and happy canning/cooking.