My 07 FXDC has a small glass window in both front and rear brake reservoirs, which should tell me if the reservoir is low or what...
How it works? That is: what should it look when it's ok, and what should it look when it's low on brake fluid?
A picture perhaps would help...

 

Should look lit up green if low.

 

So: black as if it were a hole on nothing is "ok".
Lit up or lighter color is "truouble"?

 

PSST, take the lid off, n LOOK. "F" the sightglass. Ain't never gone by them, since they put them on there. I can pull the top, faster than peeps can 'see' and 'finger out' what is what. AND evidentally, you could, too.
FWIW, IF you are running 3 or 4, vs 5, should be flushing it a MINIMUM of every 2 yrs. My book says 'YEARLY'. (HD Service Manual - FLH 70-78) 3 and 4 are hygroscopic, they ABSORB water. NOT good for the system, OR your braking. IF you decide to switch, FLUSH it really really good.

 

Thanks Gumpmeister. I know about the brake fluid (it's dot 4 here).
The lid is that of the rear brake. It's quite awkward to take off...

 

There is no light, it is just a simple piece of glass.
You should see brake fluid in the sightglass. It may be hard to tell when it is full, but it will be an even color and you can tell there is liquid behind the glass, it should be almost clear but with a slight tinge of color.
If it is low you can see the top of the fluid.
If it is very low or empty you will see no fluid behind the glass.

 

The window in the lid is not just a window, it's a kind like a dipstick, with a stem and a cone shaped appendage.
I suppose it works based on refraction, but don't know how it looks.