One of these controls is a pedal valve by which the operator of the trencher can quickly disengage the friction discs from the reaction plates by relieving {HEAD ~ all presure} on {TAIL ~ the piston} 
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One of these controls is a pedal valve by which the operator of the trencher can quickly disengage the friction discs from the reaction plates by relieving { HEAD ~ all presure } on { TAIL ~ the piston }
OTH OTH OTH OTH OTH OTH OTH 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 HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL