Weird oil pressure!

[Dorkingchris]

I'm looking for any ideas about this issue, which has confounded me and at least three experienced automobile engineers!
My 1926 12/4, owned for 5 years, develops around 20 psig of oil pressure, as it should, BUT it takes a good minute for the pressure to rise steadily to this when started. Then on switching off, it takes at least 30 seconds to fall to zero again. There is no constriction in the feed pipes from pump to gauge and I have found exactly the same effect using a modern Smiths gauge connected at the crankcase. I have also checked the short pipe from pump to connector within the crankcase and have had the pump out for inspection. The gears looks absolutely normal and there was minimal wear on the end plate, which I polished out, giving me another 2 psig. The relief valve has also been checked and is in perfect order.
I have never experienced such a low rate of oil pressure rise on any other engine and cannot understand why the pressure takes so long to rise, and even more puzzling, why it has such a low rate of fall!
Any ideas out there?
Thanks,
Chris