Finding your old Crosman 766 BB gun while cleaning out the hall closet or garage can bring back childhood memories of time spent plinking away at tin cans with your dad or grandpa. Before you share the same memories with your own child or grandkid, your Crosman will probably need a bit of TLC to get it back to prime condition. If it can’t hold pressure, it’s likely that it needs new seals. Resealing your airgun can be a little technical, but with a few tools and the right parts, you can get your well-worn gun working as well as the day you bought it.
Some of the seals in your 766 are easier to replace than others, so it's important to figure out which seal is the problem. If your airgun is unable to pump enough air to fire at all, its pump cup usually needs replacement. If your airgun can pump up but loses pressure, the valve seal is the issue.
Before disassembling your airgun, gather all the parts and tools needed to complete the job. Although the Crosman 766 was discontinued in the mid-80s, it's still possible to find aftermarket parts kits to repair it. Crosman no longer sells replacement parts for the 766 directly, but the airgun shares many parts with the 2100 model, which can still be acquired directly from Crosman. To disassemble your 766 enough to get to the seals, you'll need a flathead screwdriver, a mallet, a thin wooden dowel and a 7/32-inch roll pin punch.
Before proceeding, make sure your 766 is unloaded by removing the BBs from the reservoir. Pump up and discharge it in a safe direction to release any pressurization. Once you're sure that the airgun is unloaded and not pressurized, remove the plastic housing from the front of the handguard with your flathead screwdriver. Lift the plastic cover to expose a roll pin that must be removed with your 7/32 roll pin punch and mallet. Take out the plastic plug that it was holding in place, and you'll be able to remove the air piston assembly. The pump cup is at the very end of the air piston assembly and, because it is friction fit, can be pulled out by hand.
Replacing the Seal
To access the valve seal, you'll need to open the receiver by unscrewing the three screws on the side of the airgun. Proceed with caution; there are many small parts and springs in the receiver that could be easily lost. Fully disassemble the receiver and retain all parts for reassembly. With the receiver disassembled, you can remove the pump tube, which contains the pressure chamber of the airgun. Gently remove the pressure chamber from the pump tube with your wooden dowel and disassemble it to reveal the valve seal O-rings. After replacing the broken seals, reassemble your 766, retracing the steps you took to take it apart.