Oil preasure, or lack thereof

I have just recently pretty much restored a 1960 Minneapolis Moline Jet Star.
Heads,valves, engine bored to.42 over, new pistons, rings,rods, mains etc. The machine was done by a machine shop.
Hears the deal, I bought the tractor in non running condition. Pistons Seized up!! Got the engine torn down as far as I could, and took it to the machine shop for the engine work, and did the rest of it myself.
Once I got it all put back together got it started up and with a little fine tuning it runs fine, expect for the oil preasure
Start up and engine cold it holds around 25lbs of pres. Once warms up to operating temp. The oil preasure fluctuates with the R P M's. An idle 600/700 rpm the oil preasure gage reads about 3 or 4 lbs. Bring the engine speed up to 800/900 rps oil gage comes up to around 23/24 lbs
When I shut off the tractor the oil guage drops past the 0 preasure reading on the M M gage about a 16/32 inch
Is the low oil preasure the result of a faulty gage ( the way it goes past 0 psi when shut of)
Does adjusting the pres from the pump to the motor the useing the adjustment on the side of the block be enough difference to increase the oil preasure .AT Idle
Should I change out the oil pump?
Looking for some ideas. You guys have helped me out in the past, could you try to do it again ?
This is only the 2nd tractor I've done extensive work on, the first being a 1948 Ford 8n

Thanks sorry for being so long winded
 
Start simple. Install a known good gauge. Don't need to take the dash all apart. Can just screw it on down at the oil filter and see what it does. If pressure still is low warm try adjusting it (turn screw in for more pressure). If you can't get descent pressure at idle without pressure through the roof when you raise the RPM I would suspect a large leak. Oil pump has enough volume when over idle to overcome it. Would then air test oil system with side cover and valve cover off. My first suspect would be the tubes that feed mains and cams.
 
If it was an Allis Chalmers WC--WD ect that would be a good guess, but MM didn't use can brgs in those engines. Lube from pump went straight to Mains.
MMDEL
 
I bought a Jet Star 2 Diesel that had been rebuilt and would not maintain oil pressure according to the seller. It was not a runner when I bought it.

When I removed the side cover, i discovered that the oil tube feeding the rear main bearing was not properly installed into the rear main assembly. Everything else checked out on the lower end.

It carries good oil pressure now.
 
Another thing people do at times is use high detergent oil in a gunked up engine. The result can be the sludge in the engine gets loosened by the HD oil and clogs the pickup screen.
 
Absolutely go no further until you have installed a gauge of known accuracy. From your description your gauge is out of calibration and cannot be trusted.
 
[quote=Thanks.for the info you have posted.
Just got back from my camp where my tractor is housed. Took your advise and hooked up a good gauge i got from a friend. Fired the tractor up, oil preasure went up to 28 PSI and stayed there till it warmed up. Preasure dropped to 22 PSI when it was fully warmed up and never went any lower this was at 4hundred something RPMS
Ya'all came shine ing thru again.
Thanks!!
 

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