The cylinders define working chambers in which a working medium can either be compressed by the pistons or can exert {HEAD ~ pressure} on {TAIL ~ the pistons} 
The cylinders define working chambers in which a working medium can either be compressed by the pistons or can exert { HEAD ~ pressure } on { TAIL ~ the pistons }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL