The bypass valve in the oil filter is there to open if lets say the Oil Filter was plugged up or the oil is so thick (due to the outside temp being extremely clold) that it cannot pass throught the oil filter fast enough and creats enough pressure that that opens the Bypass Valve.
Since you have no pressure the bypass valve should not be open.

I do not know how easy it is to get at the Oil Pump but I would check to see that the pressure regulating valve inside of the oil pump is assembled in the correct direction and that the Pressure Spring is OK. Is the pressure valve stuck in the open position? Is the Oil Pump Drive gear key missing or sheared off?
Next is there a seal or gasket between the Screen Assembly and the Oil Pump housing and is in installed correctly?
Is there a seal/s or gasket/s on the outlet side of the Oil Pump and are they installed correctly.
I can remember A Mechanic who worked on my Fathers car installed a gasket wrong and it blocked one of the oil passages and his rocker arms got no oil.

The same as above with the Oil Filter Housing gasket or seal.

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84 300D, 82 Volvo 244Gl Diesel