Up date---low oil pressure---doh!!
Any chance this is related to when I did the head gaskets?
What next,drop the sump and check pickup & filter.
Or just get the pump out---or both.

 

Next upate---
Dropped the bottom,pan full of sludge,but pick up filter ok,cleaned all through,put all back together,new oil 15/40 and new filter again,start up and still low pressure.
Took off pipes to oil cooler,blew air from compressor,nice and clean.
Mmmm,dismantle the front to get too the oil pump,pan off again,and there's the problem,one full crack in the outer ring gear of the oil pump. ( Guess it was always on the cards,but one can always live in hope for an easier solution)
Ordered a new pump kit from Atlantic British along with a 180 thermostat.
Thanks to all the advice from members of this great site.

 

good luck

 

People have mentioned the oil pump gears can crack and this has an affect on oil pressure.

As long as you put your rocker arms on correctly on each head. This is where the pressure comes up to feed the rocker arms.

ah and if you were fiddling with the tappets which are pressurized by oil.
Is there a loud tap tap now?

OK re-read and see you have a crack in the ring gear. Well, you'll get it all sorted then.
You now know the problem.

 

Nice work. I enjoyed reading this thread as you sorted through your problems step by step, like a murder mystery. Thanks for the follow up as I'm preparing to do the same job myself.

 

Last update-----after finding the broken ring gear in the oil pump,fitted a new one,put all back together,plus another oil filter and more new oil 15-40,started her up and the bloody oil light stays on.( Lots of bad language )
So can't get the pressure gauge tester I borrowed,so decide to order a new oil sensor with intentions of cutting the old one and braze on a fitting to allow me to get my oil gauge to fit.
Fitted the new sensor and thought might as well give it a go---------------oil light straight out,actually turned the engine off and it took 30 seconds before the light came back on the pressure was so good,plus it was cold.
I had checked the old sensor by connecting to my multi meter and pushing the switch in and out and it switched on and off.
Anyway,to all who say oil sensors seldom fail,yes they do.
On the plus side I found the broken oil pump gear which could of ended in disaster,and lots more money and time.
I have now good oil pressure,and after fitting a 180f thermostat and bleeding the cooling system four or five times the truck seem to be running on average at 195f,happy days-----at least for awhile!!!!

 

Yep, that new oil pump is a good thing in the long run.

How did you decide you had low oil pressure before you swapped the pump?