It's a load sensing valve. When the truck bed is empty and the brakes are applied hard, the bed tends to rise. The valve senses this via the spring, and decreases the brake line pressure to the rear brakes to avoid lockup. With a big load in the back, the bed stays level under heavy braking, so the brake line pressure can be higher to provide more braking power without lockup.

P/N MB587293, about $300 at the stealer. <img src="/forums/images/graemlins/scared.gif" alt="" /> Spend some time in the yards, mate!

1990 Monty LS 3.0 A/T LWB. KYB front shocks, OME rears and coils. 33x10.5 BFG Muds. 5.29, 2.85, RD110, RD46. 2 bouncys. Aisins. Monte Disco swaybar disconnects. GenII t-arms, idler arm, UCA's, V45W front brakes. 2in BL.