308 Brake bias pressure sensor

I'm in the process of reworking the brake system on my '78 308 GTS. The calipers are off and being rebuilt, and new brake hoses being installed. Since I got the car, the "brake failure" light has been on (not the parking brake light which works, I have a separate brake failure light that is on constantly), even though the brakes work, more or less.

There is a cross-over device or pressure sensor, not quite sure of its function, and wondering if that could be the source of the problem. I know that someone identified a part from a Volvo that works as a replacement, but has anyone tried servicing or repairing the existing piece? There appears to be a sensor in the top, and wondered if that can be replaced or fixed.

As a separate issue, I guess it's also possible that the floats in the top of the master cylinder reservoirs could be defective and causing the problem. Has anyone dealt with them. I have had bouts with a similar setup in the old Jaguars, and they are a pain in the neck. The floats get fluid-logged and sink, and set off the sensors. Is anyone familiar with these on the 308, and has anyone found a good fix or replacement?

Thanks,
Steve