Hold on a second. You guys actually have phantoms that move the handle forward after cocking it back? All the phantoms I've ever owned required me to move the pump handle to the forward position manually. Return spring or not, I've never seen a phantom automatically move the pump handle completely forward.
edit: My mistake, just realized this was for an undercocking kit that I haven't owned, I can't really provide any input as to that particular undercocker.