Went through this last year with 1432. I could not figure it out initially. Hunted down and changed rear wheel cylinders with different diameter ones. No change. Sent booster back to Booster Steve to have him check it. No issue with it.
Proportional valve shuttle slide to one side (and I did not know it) when I initially bled the brakes.
When bleeding the brakes in a dry or even semi dry line car (I had changed only my front to back line) what can happen and did happen to me is the shuttle inside the prop valve slid to one side (due to the pressure differential).
Always use this tool that locks the shuttle in the middle until after you get fluid in the entire system:
https://musclecarresearch.com/brake-valve-toolOh, and make sure the 'brake' light bulb in the dash is hooked up and is not burned out (like mine was). If mine had not been burned out, it would have been more obvious and saved me a lot of headaches.
Lastly, make sure to remove tool and hook your plunger/wire back up before driving the car.