I didn't put retraction springs at all as I didn't need them. When the bench is on the casters just move it around the workshop and once bench is in it's final position, just open release valve at bottle jack and bars with casters will lift up. Actually complete bench is coming down and then it stands on the feet. As I have the same bars, casters and hydraulic jacks for both; lathe bench and milling/drilling machine, I keep bars with casters on one which was moved last. In my case at lathe bench.

Cheers,
LMMasterMariner