I have the same problem on my 455GL. The sheaves are worn and need replacing. I watch a youtube channel called SV Impavidus who has done a number of fantastic vidoes on a range of sailing repairs and improvements and is well worth a watch.
He has assisted with a repair on these genoa cars on a friends boat and it looks simple although I havent done it myself yet.
In short he has used a multitool to carefully split the sheaves and remove them from the pins. He has then been able to remove the pins using mole grips. This is all without removing the cars from the track. You can replace the sheaves and tap the pins back in.
This is a link to the video.
Hope it helps