Just to drag up this thread and to help anyone with the same issue.. yesterday I had another go at getting the fork working and succeeded.
As I said, I originally let all of the air out of both chambers and refilled with +ve first. I did this several times over but always suffered the same suck down.
This time I put all of my weight on the fork while holding the valve pin in, first on +ve, then on -ve side. I then turned the fork upside down and released the -ve valve while pulling up hard on the lower legs. I repeated this cycle several times and each time a tiny bit more air would release from the -ve side.
After this the fork seems to function more normally. With balanced +ve and -ve I get about 5mm suck down.
Can anyone enlighten me as to how this fixed it? Obviously there was air trapped somewhere or some kind of pressure imbalance. Could it be to do with how compressed the fork was when I re-assembled it? I still can’t make sense of how the -ve started causing suck down at about half +ve pressure.
Anyway, hope this helps anyone else who finds this thread!
Cheers
Dan