Hi. This is a new one, at least for me.
Warning brake light on, fluid level low. Adding fluid to the reservoir, it never rises to the 'full' line. It swallows as much as poured never getting above ~10mm under the 'full' line. Has 'normal' moisture traces on the exterior of the master cylinder; no dripping nor wet anywhere. Leaves the booster to be the one wondering?
Brakes perform normal ! So far...
Anyone experienced this ?

 

I think I've actually experienced this as well. Was topped off a few days ago also, didn't have a warning light though.

 

Do you have a vacuum pump, mighty-mac? I'd pull vacuum fitting off brake booster and slip a hose down inside connected to mighty-mac and see if you vacuum out any fluid. It would almost have to be going into the booster at that rate. If it was one of the lines leaking, it would have to flow through the ABS unit and the small hoses to disappear and that would take time. You say it is swallowing it as fast as you pour, so leaking directly through master cylinder plunger sounds more acceptable.

If you find brake fluid in booster, you'd want to get it out as soon as possible, will eventually ruin rubber diaphragm inside. Once master cylinder is removed, use vacuum pump to suck as much out as possible, then rinse with water and suck that out.

 

I had this happen a little over 8 years ago when I rebuilt the master cylinder. I messed up something reinstalling the master cylinder and brake fluid was pouring into the vacuum booster. In my case I knew of a DII at a local boneyard so I made a quick run there and bought the booster off of that truck instead of trying to clean up the booster I had fouled.

It happened too long ago for me to recall exactly what I did wrong, but I remember it being face palm-worthy.

 

This is common when it gets cold look around the master cylinder area where the reservoir is feel along bottom the leak usually runs down the brake line and around area. Lots of videos on replacing those orings