VW Passat B5 FL: STOP BREAK FAULT SERVICE MANUAL Message Persistence Despite Brake Fluid Refill

For some time now the message like in the subject has been appearing more and more often. It is probably associated with a low level of brake fluid. I added fluid, the problem persists. I'm thinking about a broken level sensor. Can anyone explain how he works? There is no float sensor on the cap, the sensor probably on the bottom of the tank. Is there a float in the tank, e.g. with a magnet?

And a colleague took out SITKO and checked exactly how much fluid there is ??? and whether a colleague has a vag to indicate if there is any error.
Buddy, I would start with the rear brakes, check the pads because maybe they wiped off a lot and the piston would slide out the same in front, then I would check all the hoses for leaking brake fluid somewhere, and at the end how everything would be correct, I would vent every wheel for sure, I don't remember if your tank on top has, like in golf, a button to test the signal, maybe some dirt has broken in.