[77newell]

What are your brake pressure gauges showing? The reason I ask is that when the pressure in the system drops below some set pressure, usually 60psi or so, a PPV (pressure protection valve) closes and separates the brake systems from everything else. The "supply" pressure gauge on the dash tells you the pressure in the "everything else" category and the brake gauge with 2 needles tells you the pressure in the two brake tanks. If something in your brake system is leaking, as indicated by a drop in one or both brake needles, then the pressure in the "supply" will stop following the brake pressure down once the PPV closes.