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Topped up air pressure, still sags 20mm or so
Home fix or send away for service?
This is the home fix, which should help you make a decision:
http://forums.mtbr.com/components/ks-lev-diy-cartridge-rebuild-899659.html
Step 5 tends to be the most annoying one.
I wasted far too much time doing the home service 2 or 3 times and the damn thing always started sagging again on the next ride... my advice is to send it away
In answer to the next question : South Coast Suspension
https://www.southcoastsuspension.co.uk/ks-servicing
Mine went back to stiff. Think I paid about 50 quid. My spare time is worth more than that. YMMV.