This isn’t necessarily true, it depends on how you install the adjustable prop valve. If you just install an adjustable prop valve behind the stock combination valve it might not help at all.
Del is right on, the stock combination valve may be reducing the pressure to the rear by too much already. If that’s the case, adding an adjustable prop valve behind the stock combination valve won’t help, the adjustable valve can only reduce the amount of pressure to the rear brakes, not increase it.
If that’s the case, you’d want to ditch the stock combination valve completely and use some kind of adjustable valve.
