UPDATE drama. Ok after a winter of riding road I finally got the Fatty fork transplant over with just last week. New SI bearing kit. New fork boot too. Not super-happy. If anything the 'thunk' and few mil travel when locked out seemed more marked (compared to when locked out and compressed by hand when off the bike) but could be just the bike frame (acoustically) amplifying this.
So I cycle two miles - mix of road, bridleway, field, a few kerbs with a big half-grin, this bike fits so nice, is so light and the newly serviced running gear is SPOT on, snicketysnoot. Now Reminded why spent so long looking to replace my old 18" F500 - the geometry is bang-on-bespoke for me, and it climbs and swoops so well, with a quickness.
But ... that little 'clunk' from the Headshok keeps on nagging. I'd put about 120psi before setting off - top limit, but about right for my 220 lbs of mass-culinity
After 2 miles I realise that something is amiss with the travel. The fork boot looks more compressed. The damn thing is now near bottoming out just by my leaning on the bars. air pressure gone. Not getting any XC riding done. Looks like a possibly expensive long haul. Have today refitted the original forks as they at least go up and down but would no doubt work better for 120lb rider :-/
Am assuming the blue Supers need a seal kit/service. (themalcolm seems like you were right) Bike fund empty for now. Bum.
Here pictured yesterday at the point of stopping: