I know what your going through!
hi there mate, i own an elstar pit bike and it has usd forks the same as yours and yes this was a problem for me too. To sum it up for all of you the forks have oil in them and when the fork is compressed the oil squeezes through tiny holes in the plunger to the upper part of the fork. now the thicker the oil the stiffer the fork is and the slower it compresses. for the fork to rebound or return the the springs push the plunger back and the oil goes back to the other chamber. so the cure is to use a thin oil and that way the springs are strong enough to make the oil get sucked through the tiny holes called valves! the problem with this is the works will rebound properly and be springy again but it will be so soft the forks will be useless.
now if you want stiffer forks you use a thicker/heavier oil but then once the fork has been pressed down or stroked once and the oil has gone through the holes/valves the springs dont have the power to suck the thicker oil back through the valves!!!! so if your like me and want a good, stiff properly rebounding fork you need to use thick our and put in heavyer springs!! i hope that long summery helps, you may think why did they work before when the bike was new like mine did, its because the springs are cheap and they loose there torsion and just dont work. please send me back a reply to let me know if this helped!
