The Crosman 1077 is a popular air rifle known for being lightweight and relatively quiet. It is a 12-shot repeater that, when working, is durable and easy to load. A leaky cartridge indicates a new seal is needed in the CO2 cartridge. New seals are very inexpensive. This article tells you how to replace CO2 seals in the Crosman 1077.
Take out the CO2 cartridge. Check that the gun is not loaded.
Use your screwdriver to unscrew the three screws at the bottom of the stock. The stock is the entire black part of the rifle including the barrel and grip.
Unscrew the CO2 assembly screw.
Use a toothpick to gently push the old seal out.
Replace the seal and reverse the steps to put the gun back together.