On a spare bike I have a 2003 psylo fork with either the heavy(blue) or extra heavy (black) spring. I can't remember which one. The rebound has always been too quick and the rebound knob has little to no effect. I replaced the damper fluid with Spectro 10wt fork oil (32.0 cSt @ 40C). This had little to now effect on the rebound speed. It is still too fast. COuld I have not added enough oil? What else could be the problem?