Help! Uneven inking; are my rails not flat? Am I doing something else wrong?

( Sorry for the long writeup in advance; any insights/suggestions/things I should be looking for are super appreciated )

I have an old 10x15 C&P New Style, and the previous user taped the rails up. When I got it, I did the same, but it’s become a bit of a headache lately.
Anyhow, I only use the two connected roller saddles, because the third (bottommost) saddle has been worn (improper oiling by previous user, presumably), and that roller core bounces around in there and makes inking erratic - so I took that out of the equation with success.
I removed all the tape, washed the rails down and scraped off the adhesive residue, then calibrated and tested my Morgan trucks with a micrometer and roller gauge (I keep them at ~1.9” diameter, slightly bigger than my rollers, because my rails appear to sit lower than my base…)
This has been mostly successful; finally getting a more even inking with no tape on the trucks or rails, which is great.
However, I noticed one side of my polymer plate wasn’t being inked. I started moving the plate all around on the base, then flipping my base, and was able to map out on the tympan an approximately 2 inch (horizontal) strip in the middle of my base that does not seem to get ink, whereas everywhere else is perfectly even.
I’m about 95% sure it isn’t to do with the rollers or trucks, because I’ve swapped them out, re-adjusted the truck width, and no matter what I do the strip that isn’t getting inked is in the same location with regards to the base.

Is it possible that the rails aren’t flat, or (in the middle, at that 2” spot), are just a little bit more raised? Is there any way for me to know for sure? What else should I be checking/ruling out?

I’ve been reading that getting your rails machined is pretty involved and pricey, so I’m holding out and hoping that isn’t it. I also really don’t want to have to selectively tape my rails, if there’s a better cure.

Please let me know if I’m not making much sense!

- Kate

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Your consistent horizontal 2” strip that is not getting ink points to a problem with the rails on both or one side. The rails are cast iron and wear on the old presses. If they wear evenly, you can compensate with the Morgans. (They bring their own set of challenges)
With all tape and adhesive gunk removed from the rails, place a straightedge vertically along the rail. Both sides. An aluminum yardstick or the back straight edge of a carpenter’s saw will work well. You may wish to have an assistant hold a flashlight behind the straightedge. If worn evenly, the straightedge will lie flat against the rail. If not worn evenly, you will see where the rail is high or low. Do this and report back. Your findings will determine what I tell you to remedy your problem. You can compensate for even wear with the Morgans, but if you have a high spot or scallops in your rails, other measures are required.
All of these fine old and worn machines can be made to print well. You just have to understand how they were intended to work and help them do their job.

Get some ink on your shirt.