There is I have one.....search my threads, I posted a vid of mine in use on my old P38....That would be good ,I wonder if there is such a thing as a oil pressure test kit .
Relief valve is integral to the front timing chain cover, there are two of them....I am sure I had a post or thread showing them and them also being taken out and measured.That's what I though bottom end bearing's ,I will change them in a few weeks then maybe change the cam and chain .I keep looking at the print out of your how to change the bearings post it's really helpful .I will check the oil pressure and the pressure relief valve first .
This is the diagram....That would be very handy I will do a search .
It is doable with it still fitted, access can be tight mind. Also, the cap can get stuck, so remove the circlip and using a screwdriver up the filter bowl lever against the spring and you should be able to pop the cap off by adding spring pressure.So I should be able to change the outer one quite easily without removing the front cover ,I have a new O ring and cir-clip to fit.