[HO455]

Originally Posted by saxart

If it were mine, I would disconnect the brake line going to the junction at the back and see if fluid comes out strong when your buddy presses on the pedal. If you're getting fluid there, keep working your way toward the rear brakes and see how the flow is at the brake line going into the rear caliper.

It may not be the fix, but it will give you information to help diagnose the problem.

2X this. You may have a bad hose at the axle. I would start at the top of the hose. Then you have divided the system in half. Depending on whether you have pressure or not decides which direction you go next. Either way Saxart is spot on. Trace back to see where the first place pressure is present.
In my world the hose is more likely to be defective from the factory than the rest of your new components. That being said I have had several new master cylinders not work out of the box. Mostly due to debris in the bore preventing the seals from trapping fluid to make pressure.

Good luck