Do the trouble shooting with a heavy piece of equipment attached to the three point hitch.
First thing to do is remove the fill cap on the lift housing and then while trying to lift an implement look into the fill hole and you can see the open end of the ram cylinder that does the raising of the lift. If there is a steady stream of oil running out the back of the cylinder the ring/s are shot and need to be replaced.
If it is not leaking there it is possible that the pressure relief valve is leaking. It is easy to access on the Hundred series model tractor. Remove the plate on the right front of the lift housing. This is the remote adapter plate or if it has a remote valve there remove it. The relief valve is going to be the little spark plug looking thingy in one of the holes you can see when the plate is removed. Do not start the tractor with this plate off. Replace the valve. It is not serviceable. I have at times though taken the cap off and found foreign matter under the ball letting it not be able to seat. Do not adjust. If you do take it apart put the cap back only to the solder stop that held it before you took it off. Damage to the pump etc can result if it is altered.
HUNDRED SERIES HYDRAULIC TROUBLE SHOOTING
INCLUDING NAA SERIES.
First you must determine if the pump is pumping correctly.
Most of these round later pumps have two plugs in the pump The one closest to the front of the tractor and in the head of the pump is the one you use to bleed the air out of the pump after air has entered the pump. Air can enter the pump from a bad pump shaft seal and or bearings and it can also enter the pump through a leaky intake O ring or a small hole in the intake pipe. If the hole is above the oil level it will not leak when standing. Usually if it is below the level of the oil the oil will leak out slowly and make a puddle under the tractor according to how big the leak is.
If the pressure side leaks it will not make air go into the pump but will cause a loss of oil.
If it does have the two plugs the one toward the rear of the pump is for priming only. Pour in some oil if needed and put this plug back in. Take the front plug out and start the tractor and run until the oil that is escaping the bleed hole is free of air bubbles. Then shut down and replace the plug and the pump should be operable.
There is the chance that the gear that is the pump driven gear has stripped out it's spline in the center of the gear where it fits onto the pump shaft. This does happen but not often. I suspect that cold oil and a leaky pump shaft seal may be the problem.
It is hard to fix the pump shaft seal if it is in fact leaking. A special tool is needed to pull the roller bearing race cup and the old seal if you save the old race. You can get that race out by welding a bead all the way around the face of the race and let it cool and then it will come out easy as it is shrunk. Then you can drive out the old seal and the old needle bearing. Dirty job but it can be done/
If it is determined that the pump is operating right and supplying enough volume of oil under pressure to the hydraulic system then you may have a sticking unloading valve in the lift ram cylinder housing.
If the lift is jumping/bumping/hiccupping it is caused by the lift trying to hold the height constant with a leak internally of the hydraulic system. The leak must be fixed before these symptoms will go away. Common leaking places are the ram cylinder O ring/s, the pressure relief valve, the O ring on the unloading valve or a blown gasket under the ram cylinder to housing flange surfaces.
On the 53 and up Ford tractors there is a gadget named the unloading valve. It is hydraulically shifted by the movement of the control valve. The control valve does not actually move oil to the ram cylinder. The oil is directed to the ram cylinder by the position of the unloading valve. The Hundred series has the same valve in a slightly different form but performs the same function.
If and when the unloading valve stops moving then the lift won't lift until the problem that is causing this condition is fixed.
A faulty back pressure valve can cause the unloading valve to stick.
The unloading valve can stick on it's own.
The unloading valve has an O ring that can become worn until it will not allow the valve to move.
The pump could have lost it's prime too. The piston pump must be bled by opening the front head plug in the pump and running the engine slowly until all air bubbles are gone from the escaping oil. Stop the engine, replace the plug and start the engine again to see if that fixes the problem. It can!
I would first try bleeding the pump by removing the front pipe plug with the allen head hole in it. Start the tractor and let it idle as slow as possible and when all the bubbles are gone stop the tractor and put the plug back in. You will loose about a half cup usually so put something under the pump to catch the oil.
If it won't work then you must go into the lift and take out the unloading valve and clean it up and replace the O ring with the Ford O ring made specifically for the unloading valve. The old part # was NCA 836B. Don't use a substitute or you will be sorry.
The unloading valve is covered by a plate at the front of the control valve housing just above the control valve. Their is a plug over the valve that must be pulled by inserting a bolt with fine threads into the plug and prying it out with two bars. The valve can then be pushed out with a punch of the appropriate size. Don't strike it with a hammer. The O ring will be on the unloading valve that needs to be replaced.
If you will look around on the other reference pages of the N board etc you can probably find some good views of the lift to give you some idea of what you will be looking at.
When taking the lift control housing off the tractor remove only the bolts that are around the ;outside edge of the housing and do not remove any of the bolt that are inside this circle of bolts. If you encounter a bolt that has fine threads on it put it back where you took it out. The fine bolts hold the cylinder to the cover housing.
LIFT LEAKS DOWN
Install an implement on the lift and raise it.
If the lift leaks down or has a bad case of the hiccoughs it has some sort of hydraulic leak internally. The most common place will be the ram cylinder ring/s. To check the ram cylinder for leaks you can remove the hydraulic fill plug and with a good light observe the open of the ram cylinder as seen through the fill hole. The best light to use is the sun light directed into the hole with a mirror. A good bright flash light will work. If you see oil coming out the back open end of the ram cylinder the rings need to be replaced.
Hiccoughs can also be caused by a leaking O ring on the unloading valve itself. Or if there is a lot of wear in the control valve it can cause it but is much less likely than the ram cylinder rings or the unloading valve or the pressure relief valve.
Also the pilot pressure relief valve can be leaking. It can be accessed by removing the remote hydraulic valve or the valve adapter plate from the top of the hydraulic lift cover that is directly under the seat and on the right side in front of the quadrant mounting point. The relief valve is not serviceable and should be replaced if suspected to be leaking or bypassing.
ON THE NAA SERIES TRACTORS THE RAM CYLINDER GASKET COMMONLY FAILS/BLOWS OUT AND WHEN IT HAPPENS THE LIFT WILL BARELY RAISE THE EMPTY LIFT ARMS.
THE RELIEF VALVE ON THE NAA IS LOCATED ON THE RAM CYLINDER HOUSING POINTING BACKWARDS AND THE LIFT COVER HAS TO BE REMOVED TO ACCESS IT TO REPLACE.
REMOVE THE RIGHT SIDE LIFT INSPECTION PLATE AND OBSERVE IF THERE IS A LARGE VOLUME OF OIL SPRAYING/LEAKING FROM UP ABOVE IN THE AREA OF THE RAM CYLINDER TOP.