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