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