Figured I'd buy a set of inexpensive hydro brakes first then do the research on them second . Are they mineral oil or dot 4? I know they are single piston but what is the pad retraction system? Low or high pressure system? I googled them but no real information there. They seem to be on a lot if Diamondbacks. Anybody in-the-know? I paid $40 plus shipping for the pair, new.