Porsche Repair: Porshce 928 warning light, brake fluid reservoir, brake warning light

Question
Brake warning light came on. Porche mechanic thought the cause is the failed sensor which is built in the brake fluid reservoir. It was replaced. But the light still comes on. The light goes away for a while if battery is disconnected and reconnected, but it returns after driving a while. What need to be done. The car is 1982 model.  

Answer
I'm not sure which light came on (the last time I looked at a 928 instrument cluster I was checking the many dozen bulbs on the back that light it up and wondering, why) in your cluster.  

A broken brake pad wear sensor can cause a light to go on.  How are your brake pads and their sensors?

Sometimes the float in the reservoir can get hung up, you can try pressing it down several times and observe if it rises easily and fully each time.

Section 90.38 of the Shop Manual addresses some troubling shooting a few of the brake electric circuits.

I'm sure you have a good mechanic.  But, if he was paid to diagnose and repair and the symptom is still there then you should ask him to give it another shot just to be fair.