Tractor Repair: David Brown 990 1974/5, david brown 990, draft control

Question
Unable to lower the hydralic arms when the tractor is running.  Unsure which levers need to be used and their settings

Answer
Hello,

  The height lever is used to raise and lower the hitch, and the draft lever is used to adjust the sensitivity of the draft control to get the desired response depending on soil conditions and type of implement being used.  Some of these models had a black knob on the back of the tractor that could be turned to lock the hitch in the up position.  But you said when the engine is running, which sounds to me that you can lower it with the engine off.  In that case, you probably have a sticking hitch control valve.  It is hard for me to tell without more info or doing some tests.