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Help! Brake Bias Troubleshooting

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 rear OEM calipers 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 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.
2) Front passenger caliper brakes more than the front driver side. I know this from locking up that wheel consistently under hard braking.

Attempted Remedies:
1) Bled the system twice by myself, and had a shop machine bleed it once.
2) Inspected all calipers for leaks/stuck pistons.

Specific Questions:
1) Is the bolt pictured below 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) Does the front outlet and rear outlet on the Master Cylinder produce the same pressure/fluid volume? I realized after the fact that I reversed the front and rear lines to the master cylinder. Would switching them back result in any change?
3) Is the lack of rear braking likely air in the lines? The last time I bled the rears, there were no air bubbles coming out.
4) 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.

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...
Nissin Master Cylinder Arrows.JPG

P2F ABS Delete Arrows.JPG


I have seen internal parts diagrams label the "front" piston "primary" and "rear" piston "secondary", which to me does indicate some level of bias, but it is fixed and not adjustable since it appears to be a factor of the piston geometry inside the master cylinder. There is a proportioning valve inside the OEM ABS system (on 91-99 cars it is inside the ABS solenoid housing and on 00+ cars it sits under the ABS bracket), but deleting it like you did simply exposes the system to the raw pressures generated by the master piston. I have seen plenty of NSX race cars using the tilton valves like you are and they work fine. My guess is that you still have air either in the rear lines, or, more likely inside the master cylinder. I would remove it and bench bleed it thoroughly.
I appreciate the wise words.

P2F Racing also let me know that my lines were not installed properly (front and rear lines exiting MC should be switched).

This weekend I will reroute some brake lines, bench bleed the master cylinder (with it still attached to the car), and bleed the whole system thoroughly.

Also for reference, max braking took ~150ft from 60-0.
+1 I don't think the lines are installed correctly. The front lines look totally independent from the rear so the system does not have a way to balance from the front to back. I think the Tilton valve goes between front and rear lines and does not go on the rear lines only. So when you add/remove pressure on the valve it should add/remove balance from front to back. Currently the front is untouched and the rear does very little since the valve can't balance with any pressure but it's own. Let us know the correct line routing when you find out, I'm not brake expert but just looking at those lines this comes to my mind.
I am not sure if this applies or if those parts were deleted. Pnsx 2002 owners manual.JPGage 159 of the 2002 owners manual.
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I just swapped the lines out of the master cylinder in accordance with p2F's installation page: https://www.scribd.com/doc/26508810...rd=D1hansex9zGeV5Ay3x21#fullscreen&from_embed

The line for the front comes straight out of the master cylinder and goes to the front brakes. The line for the rear comes out of the master cylinder, goes through the proportioning valve and then to the rear brakes.

Most cars come from the factory with a diagonal pattern brake line setup (front left connected to rear right, front right connected to rear left) as an extra layer of safety. If one line goes out and you lose braking to the front right/rear left for example, you still have braking on the front left/rear right. This ABS delete kit reroutes the fronts together and the rears together.
I "bench" bled the master cylinder with it still attached the car and got all the air bubbles out. I can't figure out how to upload a video of the bleed so here is a screen grab:

Bench Bleed Screen Grab.jpg