T`was bugging me while I was engaged making and eating my evening meal that your attempt to prime the pump seems not to have been successful (assuming pump loosing prime might be the issue?).

Lorne I very much doubt that the oil pressure relief valve would be forced off it`s seat during the attempt to prime the pump using an oil can, in the assumption it had been rebuilt and is seating correctly in it`s housing..

My thinking is currently that Jeremy`s description of removing the oil pressure sensor a fraction of a turn at a time relative to spanner access, caused me to then imagine how much time had passed prior to him replacing the sensor and then turning the engine over.... which may have been enough time for the small amount of oil he pumped into the system to drain back out of the pump gears to drain off down into galleries with little to no oil in them for quite some time, thus perhaps the pump still failed to suck oil up from the sump... ?

As I typed I have not tried this method on a Rover V8 and on both occasions I used it the issue was resolved... Were I involved directly I would ensure that I was able to feel pressure building in the oil can pumping mechanism, and given the description of events I would minimise the time between the last pumping action and screwing in the oil pressure sensor ( if that was the access point used) by just quickly screwing it in a few threads i.e. hand tight, before spinning the engine over, thus hopefully minimising the time available for it to drain past the oil pump gearing...?

I suspect much depends upon the routing of the galleries within the oil pump relative to least line of resistance,and that is why I suspect it is important to pump as much oil in as possible and in as short a time as possible prior to closing off the galleries...?

As I typed I used oil pressure relief removal on the side of the block of both engines I used this process on, thus there was obviously alternative routes for the oil I was feeding into the system to chose to flow, regardless of that in both cases oil pressure was restored when the engines were spun over, in one case to see the oil pressure needle dropping down and in the other for the oil pressure warning light to extinguish..!

If indeed the the oil is draining off before the orifice used to introduce it is sealed off, then perhaps before stripping the pump off to grease the gears, a thicker oil might be used in the oil can, thinking hypoy or similar axle lube..?

If that might prove successful, perhaps best to then perform an oil change, and hopefully by that time only replacing with a new filter initially filled with fresh engine oil, turning the engine over to confirm pressure restored, prior to warming the engine up and draining and refilling the sump, before once again confirming pressure has been restored.... Fingers crossed..?

As before, hope this helps and does not muddy the water.... ?