After my father passed last spring I found an old S&W 78g .22 pellet pistol I hadn't seen in many years. He bought it in the mid 1970s, and it was the first "real" gun I ever shot. I looked online and was surprised to find videos and written tutorials that showed how to disassemble it, replace the worn out old o-rings, and then get it back together. Reseal kits are available for this model from several different sources. I went for the one with the best recommendations, and even though it was the most expensive, I can say Dad's S&W will now hold pressure for months at a time! I've since resealed a Daisy 777, and have another few old timers on the bench waiting for parts, including the Crosman 766 I got for Christmas 40 years ago!
I'd search a little to determine availability of seal kits for your airguns, and the level of difficulty in replacing them. It seems like seal kits are available for most popular air guns, and the price is generally pretty reasonable, especially when a gun has sentimental value.
Here are a few pics of the 78g:
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Dad's pistol along with the paper copy of a 4473 from The Fair in Spencer, MA.
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Only one of the old seals was still in one piece after removal! These were all replaced along with the exhaust valve which actually lets the pressurized air into the chamber. One simple special tool was required, and it was created from a flat bit with about 3 minutes of Dremel time.
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Pistol disassembled. This was a pretty easy process between the videos and articles I found online.
