Circles not round? Backlash problem SOLVED!

My Adventure Edition 2.0 has been serving me well for over 3 years now. Recently I noticed that circles are not quite round. I saw slight corners on them that suggested that there was backlash in the Y axis. This appears as a "corner" on the circle at the point where the Y axis reverses direction a bit late. I measured the backlash with a dial indicator and found it was over 0.012". I first measured it at the router. To try to localize the backlash, I measured it with the dial indicator probing the body of the Y stepper motor. It was still there, ruling out potential slop in a lot of other parts such as the Z rails, etc. Tightening all the bolts I could get to did not help. (none seemed loose).

I thought maybe the anti-backlash nut on the Y acme rod was worn. I contacted Tom at Handibot and he has kindly sent me a replacement. THANKS, TOM!
But while waiting for that, I decided to swap the Y and Z nuts. Surprisingly, the same backlash problem was still on the Y axis. X and Z axes checked out perfect, less than 0.001" of backlash.

So I started taking the machine apart in the hopes of finding a loose part somewhere. What I found was that the Y rail slider bearings were pretty sticky. Once the leadscrew nut was removed from the Y leadscrew, the whole carriage should be able to move up and down in Y easily, but it took some force to push it.

I took off the two front slider bearings and squirted them out with PTFE lubricant, then added some light oil (like 3-in-1) to them. They move along the rails MUCH more easily now. Because the rear Y bearings are harder to unscrew without completely disassembling the machine, I cleaned them out in situ.

I see that the bearings have a grease port. Can someone here explain to us how one would use that? It does not look like something you could clip a grease gun onto (as I have done for lubing cars).

Anyway, with all four Y rail bearings cleaned and lubed with fresh oil, the carriage moves very easily in Y.
That fixed the backlash problem! It seems that all the resistance from dirty bearings was overwhelming the force of the spring in the anti-backlash nut. It is now less than 0.002" of backlash in Y.
YAY!

I cut a bunch of concentric circles ( see photo: [photos.app.goo.gl] ) and they look perfect again.

Now in addition to how to use the lube ports on the slider bearings, I would like to know if anyone has a way to prevent these from getting dirty in the first place.
I figure that a felt brush to push away dust before it gets rolled over would help. Have any of you solved this problem?