Each hole is open for {HEAD ~ a small portion} of the operational cycle to selectively tap {TAIL ~ an intermediate} and discharge pressure to the back pressure chamber 
to selectively tap
Each hole is open for { HEAD ~ a small portion } of the operational cycle to selectively tap { TAIL ~ an intermediate } and discharge pressure to the back pressure chamber
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH