Biggest problems I saw with grease guns:
1. Air trapped in the piston area of the grease gun pump; pump loses prime. Theoretically, the big long spring should take care of this (but often it doesn't.) If you can hand-pack the piston area of the pump, and then eliminate the air in the rest of the gun, it "should" be a solved problem.
2. Folks had a gun set up for bulk grease, but used the cardboard-case grease cartridges instead--or--the other way around. Set up for cartridges but used bulk grease. This will NOT work properly. The seal on the grease gun is directional, and it gets flipped forward for bulk grease and backward for cartridge use.
I agree with buying old equipment at yard sales, surplus auctions, etc. I went crazy about seven years ago trying to find a made-in-America grease gun; and NOBODY sold one new anymore. Best I could come up with was a truly over-priced unit made in India; everything else was made by the Damned Communist Chinese. I'd have HAPPILY bought old 'n' used American equipment, but I was in a time-crunch.