It's a pressure regulator, "NOT" a proportioning valve. It proportions "NADA". It is the first anti-lock brake device ever to be installed on a Porsche.
These "can" become bad. Just like brake calipers, time and sitting are the greatest enemy of the Pressure Regulator. This regulator consists if steel and aluminum components. A brake system begin "hygroscopic", they will naturally attract water. Water, aluminum and steel never did really play well together (which is why we pester you like a dentist to change your brake fluid... and floss after every meal). If your teener sat for any appreciable amount of time, chances are the valve inside your pressure regulator is gummed up. What can that do? Two things:
1. It can stick in the open position allowing a high pressure lock up of your rear calipers (you can spin like a top)
2. It can stick in the close position not allowing any fluid to your rear calipers effectively reducing your braking potential by 30%!!!
Exercise your inner DIY by clicking the "Add to Cart" button. This kit includes a custom machined, conical "seal installation tool" to help you get that impossible wiper seal where it belongs. It includes all seals and a perfect cork gasket. Special rubber lubrication helps you "get it in the bore" and new fasteners will help make even your job look (almost) as good as ours. $59.00
http://www.pmbperformance.com/diy-caliper-...ulator_Kit.html