M-5 Diesel oil pump problems

What I see in my 5 Star manual is four pieces. Install the piston first with the flat face in first. Then install the spring and if I remember correctly, the spring fits inside the piston. After the spring, install the retainer (plug) into the spring. The retainer has a pilot or reduced diameter end that inserts into the spring. Once that is all installed, you can install the retaining ring by compressing the spring to get to the retaining ring groove.

With everything else installed, I would pull the pressure line leading to your oil gauge and crank the engine with the starter without starting it. You should see oil out of that line within a minute or less if you have removed the oil filter. Sooner if the filter is already full of oil.

If you see no oil, the problem is likely inside and maybe one of several faults. I do not know how high you were running the oil pressure earlier but the oil pump drive commponents can be a weak link on some MM engines if the oil pressure runs significantly above 45-50 PSI for an extended period. Typically what breaks is either the oil pump drive gear teeth and/or the cam shaft drive teeth or the roll pin that connects the oil pump drive gear to the shaft. I do not ever recall hearing of the woodruff key shearing in the driven pump gear but assume that could also happen on a 40 year old tractor. There is also the chance that you split the oil filter canister due to the over pressure.

If none of these items are damaged, there is a by-pass piston inside the pump. I have never had one stick but would assume that if it was stuck in the by-pass position, you would not develop much oil pressure.

Good Luck! I hope it is just a regulator problem you can fix without dropping the pan. Report back and let everyone know what you have to do to fianlly fix it.