I was unaware that there is one on any XJ. And I don’t know why it’s there. The equal bores in front and rear master cylinder (not stepped bore) produce proportioning of about 30% rear 70% front due to design differences in total piston area (adding up 3 or 4 per wheel front; 2 per wheel rear) and rotor diameter (larger in front).
Some models (also S3 E-types) have a PDWA (pressure differential warning activator; also sometimes described by the acronym PIA ) that illuminates the brake indicator light when front and rear pressures differ, e.g. if there’s a leak. The same light is illuminated by low reservoir fluid level or application of the hand brake.
PDWAs, proportioning valves, and combinations of both look alike and are plumbed into the brake pipes in the same way. It has been argued that the PDWA is redundant because front/rear imbalance is revealed by much more pedal travel and illumination of the warning light due to loss of brake fluid in the reservoir. IMHO.
I don’t know, was the car originally manual brakes without a vacuum booster? If so, pedal ratio needs to be checked; power assist usually are in the 4 to 1 ballpark, manual are 5 or 6 to 1.
Calculated by the length from pivot to center of pad vs pivot to booster rod.
American cars that were offered with and without power brakes sometimes had two holes in the pedal, upper for manual, lower for power. Booster brackets were angled so pushrod was aligned.
If you decide to swap out anything in the end you need to change the brake fittings to metric and I would just take the other axle as a complete unit, and somehow adjust the ride height since the S3 is heavier and likely has other springs especially with AC.
**
A warped rotor (disc runout) will not cause dragging brakes - only substantial pad pressure can do that. Run-out will just kick the pads further from the disc - which increases pedal travel before the brakes engage.
As the pads moves away from the discs there can be no drag. But a delay in piston retraction may be caused by the vent holes to the reservoir not being fully cleared - possibly due to incorrect pushrod adjustment…
) that illuminates the brake indicator light when front and rear pressures differ, e.g. if there’s a leak. The same light is illuminated by low reservoir fluid level or application of the hand brake.