When you pump your Crosman 766 you can hear air coming out of the magazine and it will not hold air. What is wrong with it?

You have blown seals or O-Rings. The rifle was made from 1975 to 1982. Crosman no longer makes parts for this airgun. BUT they offer a service to locate repair shops that repair the rifle. See the link Below.

But first consider this. IF the rifle was in working order AND it was in perfect shape it would be worth around $50 to $60. Is it worth repairing? It would cost you almost that much to have it repaired. Today you can purchase a brand new Crosman 2100 ( almost Identical to your old 766 ) for $60 through "Pyramyd air" See the link below.

It's up to you.