I've yet to see a pencil injector that can be truly rebuilt. We regarded them as 'throw-aways" at the shops I worked at. They can be cleaned when plugged but no new parts are available to renew them (valve and seat area.
When an injector wears - it's the valve face and seat that gets hammered. On rebuildable injectors - you can buy a new "tip" an install it. On a pencil injector - no such thing available.
New ones are around for $50 each. I have no idea what some places are selling as "rebuilt" and what has actually been done to them.
I already asked that question on another tractor forum and was more-or-less asked to leave the forum. That was Lavoy's forum. He sells "rebuilt" pencil injectors and took offense when I asked him technical questions about what process was used to renew them.
Maybe somebody somewhere has found a way to actually regrind and narrow the valve and seat in a pencil injector - but I doubt it. Considering that the Chinese putting brand new ones on market for $50 - I can't see how trying to rebuild can be cost effective.