The rear brake line goes into valve mounted on the frame, over the rear axle. The valve "core" is turned by a lever attached to a rod, other end of rod is fixed to axle. As the truck is unloaded and frame travels up from axle, valve closes off, and rear brakes are reduced. (Why???

On a truck with ABS no less?


)
Transversely, you get more brake on rear with a load on the truck.
I've been aware of this little device (wish I knew he correct nomeclature. . ) for a while, never really messed with it. Had the rear brake line blow last fall, so I thought I'd replace the whole mess back there. I ran into problems with the intermediate hose between this valve and the axle. Valve end of hose has a different fitting. That hose is (was) not available, not even at dealer. I
could easily get a hose for this truck that would hook direct to the steel line. (it was for an ABS option?? dont remember what spec. was different. ) I had to have the truck back on the road, only way was without valve, so I crawled under, unbolted the valve, and threw it at the scrap bin. New lines on, brakes work great.
I did notice the pedal feels a little different, but my aging front rotors dont fade and the truck doesnt nosedive so bad.
As far as I know this valve thingy is common to hydroboost systems with rear drums. Something like '97-ish to Y2k, or '01. ?