I have a 68 FHC 2 +2 and the low brake fluid light has always been on after I got the car running after sitting for 33 yrs in my parents' garage. I know I could buy two new sensors for about $130 but curiosity got the best of Mr. I decided to see if I could pull one sensor apart to see what failed.

After some persuasion, I got the whole sensor completely apart and discovered a really heavy piece of cork at the bottom of a rod that triggered the sensor.

One option was to replace the reservoirs with one from a Mitsubishi. However, it seemed that replacing the cork would work.

I went To a local industrial hardware store and found 2 corks that were just a little bit longer than the ones on the car. I trimmed the corks, removed the old heavy one and twisted on the new ones. I then assembled the sensor and installed on the car. Voila!!

The red low brake fluid light was not on, and the testers at the top of the reservoirs both worked.

 

A common failure. The cork eventually absorbs brake fluid and eventually sinks. A simple wine cork will fix it too if you really want to go the cheap route. More modern repo sensors use a metal float - not original but should be a permanent fix.

 

That was my understanding in my research. I considered a wine cork but then I'd have to drink two bottles of wine, which would be a challenge for me, not to mention any damage caused my removing the cork. They were only a few dollars at the hardware store. Would be nice if there was a metal or plastic replacement for the original sensor. I imagine the cork is good for a decade or two.

 

This was a great post and helped me out a lot, thanks.
I have repaired my unit a different way to avoid having to remove the top of the unit.
My repair was as follows:

1. Cut out the bottom of the float unit (which alleviates the need to disassemble the top). I first used small wire cutters and then finally trimmed it off better with small scissors. The aluminium casing is very thin and can easily be trimmed and cleaned up with scissors. I cut around the line where the two holes are already in the side of the unit to allow a good starting point
2. I gently pried open the claws on the bottom tab of the old float to allow it to slide off the shaft and removed the old float and replaced with a new float from a wine bottle. I trimmed the new cork down to the same length as the old float and reinstalled and it all works well. I considered using something other than cork but did not know if this would react with the brake fluid and as the old cork lasted many years I decided to use cork again
3. Below are some photos of this work and I hope this helps