I was under my 1989 2500 suburban this past weekend. I was following the rear brake line from the rear axle forward and found that the hard line runs through a wierd valve that is linked to the rear axle housing? Was this some type of early attempt to minimize rear wheel lock up? I can find any mention of it anywhere. My LMC truck parts catalog pretends it doesn't exist. What is it? My other question is, can I bypass it? Or is this critical to the safety of the truck's brake system?