I¹m not sure I have the plumbing parts to connect to the input of the combination valve. I¹m certain I¹m testing the same line. The output of the MC and combination valve for the rear brakes is a 9/16 connection. I guess the line may have an issue from the MC to the valve but does not seem likely.
Other than the pressure differential switch, the combination valve for disk brakes is just a straight through pipe to the rears, and a "tee" to the fronts. The pressure drop you are seeing is in the line to the valve, or the valve. If you are pretty sure it isn't the line to the valve, then it has to be the valve. Replace it.
OTOH, nag warning, you are measuring two different lines.