this seemed to really help me understand a hammer valve and how it works better maybe it can help newbies
http://www.zdspb.com/media/tech/animations/omen.gif
(hammer valve is above reg)
this to and its a simple pump design
http://www.zdspb.com/media/tech/animations/pgp.gif
and a good spool design
http://www.zdspb.com/media/tech/animations/automag2.gif