Crosman has been producing air guns and archery equipment since 1924. The Crosman 766 American Classic was a variety of the Crosman 760 family; it was one of Crosman's largest sellers. These air guns were made with three seals that served to keep the chamber air tight. The seal often becomes dry or worn, and will not hold air adequately. The seal needs to work appropriately for the gun to fire.
Unload the weapon completely. Visually inspect the weapon to ensure that no ammunition is present.
Unscrew the hand-guard pivot screw located at the forend of the hand guard. Remove the two retention-screws located at the hand-guard centre. Remove the hand guard.
Unscrew and take off the pump guide-arm. Unscrew and pull out the air chamber.
Remove the detent pins and the plugs from the front and rear of the air chamber. Hold the air chamber vertically, and slide out the compression piston.
Separate the compression piston and the three seals. The piston is unscrewed in three pieces, and has three leather seals. These are located in the centre of the piston.
Replace the seals if broken or rotten. Soak the seals in lubricating oil for 24 hours if they are intact. Wipe off the excess, and replace. Reverse the above process to reassemble the air gun.
Pay close attention to your Crosman 766 as you remove parts. This will save you a headache when you have to reassemble. A Crosman seal kit is useful in seal repair.
Never attempt maintenance on a loaded air gun. Never point an air gun at anything that you do not intend to shoot or kill.