[HO455]

What parts have you not changed since you started? I couldn't see where you changed the rear hose since you started the thread. You said it was new and you inspected it, but have you done your pressure test at both ends of the hose? Yours wouldn't be the first new part to be defective. I have seen hoses that work when straight or at low pressure, but kink up internally under pressure or bending.
Good troubleshooting isolates the problem area. Define the problem. Verify the source as pass or fail. Then spilt the system in half and verify pass or fail. Keep dividing the system until you have isolated the failure point. Don't assume any part is good until you test it under load.
So you have good pressure at the caliper. FALSE
You have good pressure at the master cylinder. TRUE
You have good pressure at the outlet from the proportioning valve. TRUE
Next step is go to a convenient pressure test point between the proportioning valve and the caliper. Like the hose connection to the hard line at the frame. Or at the axle end of the hose. Or at the double male connection under the passenger seat floorboard.
Good luck.