[Tom, F&L GoR]

As part of the Federal Motor Vehicle Safety Standards, all cars in the US are required to have a proportioning valve. The prop valve is actually a pressure limiting valve designed to prevent brake line pressure in the rear from continuing to build at the same rate as the front in maximum braking events. The idea is that with more weight shifting forward in a braking event, the rear end gets light and cannot accept as much braking; therefore the rear might lock up and the vehicle loses direction stability. All this is what PhoenixGTS just said.

The "problem" with Vipers is that the brake balance of four piston Brembos up front and one-piston Brembos (same brake design as a 1988-92 Renault Alliance) in back was never close enough to cause rear wheel lock even without the pressure limiting valve!

The change to 40mm rear brakes a large difference, part of which is eliminating that predictable and terminal plowing feeling. Because it is "different" I recommend that folks first leave the proportioning valve in until they get used to this, then experiment with high braking and the valve disabled. The slight increase in rear brakes gets the front-rear balance that much closer, will again change the "feel" of the car, and if the driver likes how it "feels", can perhaps slow down quicker. While I know many have removed it, myself included, the difference between the prop valve in or out is a driver preference issue; different racer types have it in or out and different street drivers have it in or out. In ~three years (wow), I've not heard anyone that did disable the prop valve go back and reinstall the spring or seal.