A rebuild kit with new exhaust valve, pump cup, check valve and o-rings will run about $25. Replacing all the components will probably take an hour to 1.5 hours, if you take your time disassembling, cleaning, and lubricating as you go. There should be a few videos on Youtube that show the process.
A few years ago I dug out my old Crosman 766, which is almost identical to your 2100, and found it wouldn't hold pressure. I bought a reseal kit and got it back up and running like new. There's something nice about shooting the actual rifle you got for Christmas back in 1980!
View attachment 892808
The inside is pretty simple. No special tools are required for disassembly/reassembly.
View attachment 892807
The brass part is the valve body. The white part on the left is the exhaust valve that lets air into the chamber when you fire the rifle. The white part on the right is a check valve that holds pressure in the valve body while you're pumping the rifle up.
View attachment 892806
Shooting the 766 on Christmas Day 2022, some 42 years after getting it as a Christmas present!