I've run into this milky discharge from a maxton valve during leveling before. It was causing inaccurate relevels, and poor stopping consistency. It ended up being a bad o-ring under the up block. Installed an o-ring rebuild kit and a set of new stems and it went away. A second similar situation a few years back revealed a crack in the custom welded pump pipe just before the valve inlet. Some hack-ass smaller outfit here in socal installed a uc4 in a dmc where an I-2 used to be. I know you're all thinking yeah! those old dover valves blow, id