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Where the lift arms were up or down had nothing to do with the lift getting weak. Probably the 134 oil made a leak show up that did not show when the old thick 90W oil was in it. The 9N lift has no position control.To maintain the lift at a given position the hand must constantly be on the control lever to continually re adjust. I can fix that for you if you will Email me at wzsherman@aol.com or see the link below. The most common places to leak on the lift is the piston rings and the pressure relief valve.
In order to determine where the lift is leaking you should remove the right hand inspection plate that holds the dip stick for the lift oil. If oil runs out when this cover is removed it has too much oil in it and letting it run out is ok. Start the tractor with a load on the lift arms such as a rotary cutter etc. raise the lift and observe inside the lift to see where the oil is leaking down. If the piston rings are leaking their will be a pretty good stream of oil coming out the end of the ram cylinder. If just a few drops this is ok. If the bore of the ram cylinder is not badly scored a new set of rings and a little honing will fix this. I would recommend a piston and the Neoprene rubber ring for a badly scored cylinder or if you really want to eliminate leak down completely. Observe the oil in the reservoir and if you see the oil swirling around the relief valve is probably leaking. It is the gadget that is screwed into the back of the hydraulic pump just under the pto shaft.It will be under the oil and feels something like a spark plug. You can remove and replace the relief valve with the PTO in place by bending a boxed end 11/16” end wrench about 60% so that it will reach under the PTO and still be long enough to give some leverage. A cut off wrench will work too but will have to be hammered to loosen the valve. If before starting the tractor you will put your hand into the oil and locate the relief valve you can reach in and put one finger over the end of the relief valve and determine if it is leaking after the tractor has again been started. DON'T MAKE THE MISTAKE OF PUTING A FINGER IN THE PUMP MECHANISM!!!! If however you don't see a significant leak from the top you can assume that the relief valve is leaking and simply replace it with a new one and thereby prevent the possible loss of a finger or two. I;ve never tried it but I’ve read that you can drain enough oil from the lift reservoir to uncover the relief valve and see if it is leaking instead of using the obviously dangerous finger feel thing I described.
There could also be a blown gasket somewhere in the upper portion of the lift but if so you will see a lot of oil coming down from the top and then it’s just a case of taking the cover off and finding the culprit. The pump gasket can also blow inside and when this happens the lift will not raise any weight to speak of and is just about impossible to detect. The only thing to do in this case is to eliminate everything else and then pull the pump and replace the gasket to differential housing gasket.
Be sure to release the control rods from the control valve T on the pump before trying to remove the lift cover. To fail to do so will result in a broken lift control valve stem. If the control valve is stuck the lift will not raise or go down if it is up. If you should be having trouble with the lift not going down when the lever is moved to the down position I would suggest that while the inspection plate is off on the side of the lift housing you reach in and manually push the two levers that go into the oil in the reservoir and are connected to the control valve. The control valve sometimes sticks in the up position. If you should find that you can make it go down by pulling the valve toward the rear of the tractor internally on the control rods then you should drain the lift and pour about 5 gallons of clean kerosene through the lift with the drain open. It is best to pour it directly into the inspection hole. Let the kerosene that drains out sit for about 15 minutes and pour it through again leaving the portion that is in the bottom of the container to be discarded. You can also do a pretty good job with a garden sprayer with kerosene in it or diesel etc. A pressure washer will do it too but then you are going to have the water residue to contend with. You can never get it all out and it will discolor the oil from the start up. You should also inspect the spring that is connected between the front end of the ram cylinder of the lift to the knee portion of the control rods that go from the linkage at the top to the control valve in the bottom of the pump. This spring sometimes looses it tension from age and usage and must have enough tension on it to pull the control valve out when the lever is moved to the down position.
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