mm wrote:
What's with my grease gun?

Don't know, can't see it from here...

Is lithium grease what I should be using, say for tie rod ends and
ball joints?

That's fine...

If there is leftover brown grease in the gun already, do I have to
clean every little bit out to use the lithium, or will it mix together
all right?

Depends on what the old grease is...lithium and moly generally won't
mix well and can turn into a non-lubricating mess with certain types.

I cleaned it pretty well, put in a lithium tube, and after I released
it, somehow the plunger when almost all the way down. Is the new
grease too thin for the old tube, and now all the grease is behind the
plunger instead of being pushed by it? ...
....
So I got a tube of lithium grease. It wouldn't go in until I took the
plastic cover off of the one end, so the grease gun diameter isn't
very excessively big. I pulled out the plunger and latched it, put in
the tube, screwed on the head, released the plunger, and it went part
way down. I pumped the lever, and when nothing came out, I pushed on
the plunger a bit and with almost no effort on my part, it went half
way down. But still the lever worked just as badly. I pushed a bit
more and the plunger went all the way down. I pulled and it came all
the way back.

Did you also remove the seal from the other (rear) end of the tube?
Could be the pump mechanism wasn't working was why the gun was in the
garage sale to begin with.

The rod (on most decently made guns, anyway) _should_ go on in, it's
not rigidly attached to the plunger, only on the front so it can pull
it back for loading.

Could also be dry grease has blocked the exit somewhere, may simply
have an air pocket you've got to pump out, ...