[142 Guy]

Full disclosure, I have a 1971 142, not a 164 so no direct experience. But, I expect the systems are similar.

First off, I have never touched the little bolt on the MC.

I highly recommend using a pressure bleeder for the Volvo dual diagonal brake systems. I have had 3 Volvos with that system which originated with the 140 and it makes bleeding a snap compared to the alternatives. The pressure bleeder eliminates the need for that pre bleeding of the MC that a of people like to do.

Did you rebuild the master cylinder or is it new? I have had no luck rebuilding MCs, ending up with leaking past the primary piston seals back into the reservoir.

If you removed / replaced the MC check the push rod adjustment on the brake booster. The service manual specifies a clearance between the front tip of the push rod and the back of the piston in the master cylinder. You will need a depth gauge / vernier caliper to do this measurement because you can't check the clearance when it is assembled. If you have excess clearance between the push rod and the MC piston you will have 'lost motion' and will not be able to generate as much brake system pressure.

I presume that you have checked for weeping around all the brake fittings and are not experiencing a drop in reservoir level after doing your testing?

The Volvo system can be a royal pain to bleed. I expect that the likely cause is residual air in the brake system. A pressurized bleeder tool is much more effective at moving trapped bubbles quickly through the system than the foot pump method.