I bought a used Crosman 1077 air rifle. It had a CO2 leak at the inlet. Turns out it's a part # 38-128 "End Seal" available from Crosman for about $6.50 including shipping. There is no other part I can find locally that will get the job done.
Seems air leaks in Crosman 1077s are commonly reported and people complain about them on forums and just assume the rifle is junk. Look, when the valve opens on the CO2 cartridge then CO2 turns from liquid to gas and gets very, very cold. It is no small wonder that the seals will freeze or crack. There are two places where these rifles tend to leak. The first is at the CO2 inlet as I have found with my rifle, and the other is in the valve. If you take out the three screws holding the rifle in the stock and remove the rifle from the stock, then it is very clear where it is leaking. Either it is leaking right at the tip of the CO2 cartridge, viewable in a little "peep" hole and easily heard when the rifle is out of the stock, or it is leaking up between the barrel and valve where you can see in the magazine well. If it's like mine, then it's a very easy fix, not even requiring disassembly of the receiver. If it is leaking in the valve, well the easiest thing is to order a new valve.
This disassembly guide will help.
A bunch of other CO2 guns use the same seal and will have the same symptom. Happy airgunning!