It sounds like the hydraulic hose to the rear lift arm piston is leaking. Not a difficult task as it goes from the transport valve to the piston(which is inside the pumkin. It is a good time to replace the cylinder seals at the same time. Replaced the one in my D15 myself with the aid of a cherry picker shop hoist a long time ago. Before you remove any bolts, drain the differential and hydraulic resevoir. I think you need to remove the brake linkage rods and the line at the transport valve that goes into the pumkin. Then remove the pumkin to expose the piston and remove it and replace the seals in the piston (available from any of the fine AGCO suppliers on this forum). I recommend that you also get the AGCO hose from them also, the Allis has a high pressure hydraulic system and most after market ones may not hold up...and the correct length is real important.