g'day Rc1700.
I have read all the responses to your post some are misleading and some are totally irrelevent and could send you down the wrong path ,the KISS approach is good advice always begin with the simple things first , Change the oil and flush out the transmission with Diesel or whatever your local oil supplier has making sure the flushing fluid is thoroughly drained ,leave the two drain plugs and the filter cover of overnight then reassemble and fill with the correct weight oil ask your supplier .
Now you know whatever goes through the pump will not cause damage .
The impact screwdriver yes but I am answering your question considering I do not know your depth of knowledge of things Mechanical.
The diverter valve mentioned on top of the housing, Your tractor does not have one fitted.
The mention that Massey hydraulics are simple ,If I had a dollar for every tractor that arrived in my workshop ,in pieces I would be able to buy myself one of those Fancy Dodge Rams wih a BIG cummins Diesel ,Only joking I ask that for your sake do not start pulling things to pieces I am only to willing to advise as you go along ,as I have mentioned in my previous post ,remove the cover and tell me what you see (send me photographs and that will confirm what series pump you have) the more information you feed back the easier it is to help .
Happy Days.
Hutch.