Mine (1961 FSM)has a diverter valve on the top of the hydraulic cover, with a knob that when pulled out isolates the rear lift arms and then sends fludid out the port to the spool valve that operates the hydraulic rams for the loader.
Return oil goes back to diff. Return hose is plumbed into oil fill cover at back of diff.
Did find diverter valves for the Massey Ferguson 135 and 178 on Ebay, but have not come across one for the major.
SPARTEX may be able to help.
There is a port untop the hydraulic cover that you could plumb into and operate a spool valve and hydraulic ram, only down side is without diverter valve your release valve is constantly working as your rear arms will be activated and the release valve will come into play.
Hope this helps.