Having discovered that the master cylinder cap plus level sensor is no longer available I set about making a cheap replacement. I had a quick look through brake level sensors on fleabay and decided that the 3 Series items ought to work. It?s a nice sealed switch unit using a magnetic float and reed switch. I purchased one for the grand total of ?3.50. It was very easy to pull apart and using some surplus PTFE stock I had, I turned up a simple adaptor. The switch is a tight interference fit into the PTFE so no seals are required there and a large silicone ?O? ring is used to seal between the adaptor and the body of the reservoir.
Thank you for this write up.
I have been looking for an elegant solution rather like that for remote reservoirs (historic racing use).
It is surprising (yes, I am rather hoping to be corrected 
) that AP Racing, Tilton, Alcon, etc are not really “switched on” to this type of safety equipment.
After looking around, the best solution from the 60s seems to be the Jaguar E-Type remote reservoirs although I think these do not have screw in connections and that is a source of concern IMHO.
Andrew
the sensor (low level switch) or SOVY unit are readily available through RD Ent for $61 US.
it’s the CAN that is no longer available…
Have you seen the Compbrake fluid level switch on EBay. This could be a ready made solution.
Looks like a good idea. I have seen others from Car Builder Solutions as well. Many thanks for that tip !