Brake Fluid Reservoir Question
 

W123, W126, R107 and certain other chassis from the 1980s were equipped with brake fluid reservoirs that had two depressable black rubber "caps", one for each brake circuit fluid chamber.

What was the function of these "caps" and why were they eventually eliminated?

The late models with a single cap has two chambers in the tanks.

jack

If I am correct about the caps that you speak about, these were switches that turned on the brake light when the fluid was low.

Have a great day,

if i am not mistaken these buttons are used to reset the low fluid warning light that comes on when your brake fluid is low, or when you change your brake fluid all together.
good luck

If I'm not mistaken, these buttons can be depressed on some models to test out the idiot light system (i.e., push on the black button, the idiot light for low brake fluid on the dash should come on.)