Proportioning Valve Retrofit
The NA6 Prop valve has a low 'knee point' - this means that the car has a strong front brake bias, and so the front brakes are more prone to early lockup while the rears are doing less work than they could be doing. Having upgraded to NA8 brake hardware, and with a lowered car, more rear brake bias was desired.
An adjustable prop valve (eg Wilwood, Tilton) would be cool, but too much work to install (hard to find the right adaptor fittings) and dial in properly. Plus it would be just another thing for me to fiddle with, which means I'd probably get it wrong, and too much rear bias is a Bad Thing.
So I chose a prop valve from the NB model, which has a higher knee point. The part number is NC10-43-900A. They're not cheap, but installation is a snap.
Drain the master reservoir first (with a syringe), then unbolt the old valve with a 10mm spanner and remove.
The original valve is on the left, the new one on the right. The fitting locations are all the same. there's an extra bolt hole, but nothing on the car to attach it to. The outlet ports are bigger OD, which means...
...that this tab on the bracket gets in the way. Just bend it down.
All done! You'll need to re-flush the brake fluid to get the air out of the lines. Results? slightly more braking power under harder braking. The rear brakes are now being put to more effective use, and the fronts are less biased to locking up too early.