The switch opens {HEAD ~ a solenoid valve} which vents {TAIL ~ the hydraulic pumps} which move the tracks of the trencher in the forward direction 
vents
The switch opens { HEAD ~ a solenoid valve } which vents { TAIL ~ the hydraulic pumps } which move the tracks of the trencher in the forward direction
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH