The valve underneath is the proportioning valve. IF it has a brake circuit warning switch installed (I think that some don't), the switch must be unscrewed and a dummy plug installed while brake bleeding, because the warning switch is activated by a differential in front/rear brake pressures- it does this by having a sliding piston setup in the valve that gets pushed one way or the other when brake pressure varies between the front and the rear systems. The piston has a notch in it where the tip of the switch engages .... If you are bleeding the system without a dummy plug, the warning circuit piston will move out of position and not return to neutral, causing the brake warning light to remain "ON" all the time (although our old trucks are not wired for brake warning lights).
