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Help! Brake Bias Troubleshooting

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24 March 2020

I've done a brake overhaul on my '91 NSX and could use help troubleshooting. It had APR calipers up front with OEM calipers in rear and a semi-functioning ABS unit. I recently installed a Stoptech BBK for the rear (after powder coat), reinstalled APR BBK up front (after powder coat), and installed a P2F ABS delete kit.

Current Symptoms:
1) Front passenger caliper locks up (read brakes more) before the front driver side.
2) Front calipers appear to be doing all of the work with very limited rear braking. I tested this with the car on jack stands and the fronts engage before the rears do. I adjusted the Tilton proportioning valve all the way open to try and maximize fluid pressure to the rear, but experienced little/no improvement.

Specific Questions:
1) Is the bolt in the middle of the Master Cylinder just a plug? I could not find anything on the internet or in the service manual that this does anything, unless it is somehow used to adjust the piston to pushrod clearance. I was fantasizing that this somehow adjusts bias, but I'm sure it doesn't.
2) Do the two outlets on the Nissin Master Cylinder output the same fluid volume/pressure? I realized after the fact that I hooked up the P2F ABS delete kit backwards in relation to the master cylinder. Maybe this has an effect?
3) Does anyone know of a way to troubleshoot the proportioning valve? I adjusted it from full open to full closed and it didn't seem to greatly affect braking performance.
4) I've bled the system twice myself and had a shop machine bleed it once. Is it possible there is still air in the rear brake lines?

I could use a braking wizard to help me troubleshoot. Pictures of the setup are below. I am fully prepared to be roasted on my silly brake setup, but I assure you, reusing old parts was the original plan...