Only 2 things can go wrong with the rod: bent, and you'll see that quickly, or severely worn. Extend rod fully and visually inspect for worn areas or nicks, scratches, dents. These will destroy a new seal quickly! Another possibility is that whoever rebuilt cylinder installed wrong seal or installed seal backwards. That pretty much covers external leaks. Problems with internal are similar but cylinder needs to be dismantled to inspect.
If you truly do NOT want to use the cylinder as is, I'd remove and inspect. You may have to replace $10 worth of seals or the entire cylinder, but until you inspect, you don't know which. Bob