73 Manual Disc/drum Master cyl no residual valve

That wouldn't reduce pressure, just flow in both directions.
I guess his would allow the rear brakes to come in a little later than with the larger holes and if the bias was just a tad off, this would work.
From what I've been reading a proportioning valve remains open until a set pressure has been met then closes off completely much like a fuel pressure regulator behaves.
Exploded diagrams seem to support this.

In theory one could reduce the main spring rate to reduce the pressure required to shut the valve, but again, unless someone has a matrix on what spring rate equals what pressure the testing time required to mod the original would quickly overrun the cost of an adjustable valve.

My adjustable valve should be here Monday