a spool provided in the casing and configured to switch the output port between {HEAD ~ the pump port} and {TAIL ~ the tank port}, and 
a spool provided in the casing and configured to switch the output port between { HEAD ~ the pump port } and { TAIL ~ the tank port } , and
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH