What does this do?

I post this only as an explanation as to what they do.

http://www.jegs.com/p/JEGS-Performance-Products/JEGS-Residual-Brake-Pressure-Valves/948183/10002/-1


Another source,,,

Description

Forever eliminate the dreaded spongy-pedal syndrome. The SSBC Residual Pressure Valve retains brake-line pressure which keeps your brake pedal firm. A firm pedal engages quicker, giving you perfect braking response every time.

The Residual Pressure Valve works by stopping brake fluid from draining back into the master cylinder. This keeps caliper pistons and brake shoes from fully retracting so your brake system stays primed and ready for action—without brake drag.

Available in a 2 PSI model for disc brakes and 10 PSI for drum brake systems. Drums require more pressure to compensate for return-spring tension.