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