Unless you are very handy with tools I recommend one of three things.
Crosman still has parts available for this rifle and they can be ordered through Crosman. But this will require you to dissemble your rifle to locate the bad o-ring or seal. You will have to download the parts list ( See the link below) there are 10 variations of the 760 so you will have to identify which one is yours.
Crosman also offers a service to locate repair shops that repair older air guns or rifles. ( see the link below ) when it opens, click on "Find a service center."
Your last choice is to just purchase a new one for $30 ( see the link below ) For the price, this would be my choice.
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