My 20 gallon, oil less Cambell will not pump up. It does run ok. Is the cylinder wore out? Is it worth fixing?

I'm not familiar with the Campbell but it's very possible the Valves (probably REED) or worn or just plain dirty.

I would take the Heads off and have a look. It just may need a cleaning and new Head Gasket.

If it's an old Compressor and the Main Bearings appear to be TIGHT, I would go a step further with a Cylinder Hone and New Rings ;o)

Also; at the same time you can clean out the tank by dropping a few pounds a ball Bearing or Nuts into it and rattle it around; after which you can slosh it with a Muratic Acid Solution then rinse it out with Baking Soda Solution to neutralize it then Soap and Water.

Always keep your compressor in a Clean Cool Fair Sized Room Environment with plenty of Ventalation with fresh Air Filters and Oil. Small Enclosures, Dirty environments and over heating make short life of them and will cause the valves to leak.

Of course I referring to larger 5 Hp Steel Cast Compressors such as an IR but the practice is the same.

I had a problem with my Sears not pumping up as fast. It was real slow. Checked for air leaks...everything ok on the outside. It was the ring and piston. I bought a service plan and when they took it apart they called me and told me it had been full of blue coloring and asked if I was a paint/ body shop. I am not but let me neighbor previously use it to paint a pickup...guess what color...yup. The filter on the compessor is not good enough to keep out the paint if used in the same room as the car being painted. Just an FYI.
I got it fixed and I dont lend it out anymore :)