Sounds like you have some air in the system, that light only comes on if there is an un-equal movement in the two systems. In other words, if there is a leak in one system, there is one system which has pressure and the other one doesn't so the switch moves turning on the dash light. If there is no leak, air will also cause the problem if one circuit has air and the other one doesn't the switch will move and then MIGHT return as the bubble is compressed enough to equalize the two system pressures. I had that same problem on my 95 Scenic Cruser on the Spartan EC-2000 chassis. I had to power bleed them to get the air out, the normal pumping the brakes method didn't work.
1995 Gulf Stream 34' Scenic Cruiser Diesel Pusher 8344WFD, 1984 Shasta TT 28',
2019 Jeep Cherokee TrailHawk Elite,
Road Master Sterling All-Terrain Tow Bar,
Air Force One Brake System