What is the technical name of the dip stick with sensor that is attached with the cover of the brake oil container - BMW 735iL 1999 model?
I believe you’re referring to the brake fluid level sensor.
When the brake fluid level gets too low, that sensor causes the brake warning light to come on, on the dash.
Tester
And if that happens, it’s time to get your brakes checked.
That’s the system that warns the driver that his/her pads are probably low. As the pads wear, more of the fluid stays in the calipers, leaving the fluid level in the reservoir lower. When sufficient wear has occurred to allow enough of the fluid to remain in the calipers to lower the reservoir level sufficiently to illuminate the light, it’s time for a checkup.