Mike, thanks for the info. Parts are still available, not sure on prices, but Greg here on APUG works for the distributor, and has parts manuals, etc. I haven't ordered parts for the CP-51 yet but will soon hopefully. I am sure they will not be cheap, but these weren't cheap machines to begin with...
If you can run the Fujimoto replenishment unit that might be easier than the Kreonite system... just a thought. It's nice and compact.
I use RA-RT also, great stuff no doubt.
The springs are titanium I think. I have replaced them with steel springs in some cases, but stainless springs would be better - or rubber as you mentioned.
I haven't checked out the electronics on the CP-51 yet but soldering in new batteries or caps is something I'd be comfortable with if need be.
For the CP-31, I bought an entire spare system (main unit, W/D unit, replenishment unit) to keep as a spare, should I ever need it. It works fine, but I don't need 2 of them set up, and i haven't had to harvest parts from it yet, but if need be I can. Getting a whole used unit for parts is way cheaper than sourcing new replacement parts I think. I got the above spare system for $100.
I've been looking for a spare cp-51 but those are harder to come by.