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