in the slot 223 including its two extreme positions the bobbin 232 mounted on said pin grips one of the pairs of dimples 231 associated with said slot when the hand lever 230 has been positioned as described in the preceding paragraph the implement can be moved at the will of the driver between its fully raised and fully lowered positions by appropriate movement of the hand lever 88 as the implement approaches its fully raised position the pin 218 on the rockshaft 73 contacts the lever 219 and moves it about its fulcrum 220 so that said lever acts through the pin 221 the arm 222 and the spindle 211 to force the arm 210 in an anti clockwise direction as seen in fig2 into its minimum draft force position against the action of the spring associated with the slide valve 37 this is permitted by the lost motion connection comprising the pin 217 and the abutment 216 which are normally held in contact with one another by the action of the last mentioned spring but which are now held temporarily out of mutual contact so that the arm 210 is immobilised the pin 217 and the abutment 216 are allowed to move back into contact with one another as the implement commences to be lowered the purpose of rendering the arm 210 immobile when the implement is fully raised is to avoid the automatic feed back of undesirable signals that is to say to avoid undesirable movement of the balance lever 207 and thus of the slide valve 37 due to movement of said two armed lever caused by bouncing of the implement.