I just replaced my brake master cylinder because I found brake fluid where the MC and the power booster connect - like your's. The fluid was seeping down the booster housing. If it was the fluid resevoir to MC seals you would expect to find not only the rubber grommets in the condition that engatwork's pictures shows, but also brake fluid on the MC around the grommets and probably drops coming off the bottom of the MC onto the sheet metal below the MC.
A MC leak from the seal around the activating piston (at the point where the MC connects to the booster) may also deposit fluid into the booster and show some seeping on the outside of the booster (like your's). Sounds like most of your fluid may be going into the booster because it sounds like you've added a lot of fluid without any puddles. If the MC is leaking a lot into the booster, then I suggest taking the booster off and draing the fluid out then cleaning the remaining fluid out of the booster. Inspect the booster's rubber diaphragm for damage caused by the brake fluid.
My last two MC's leaked where this seal is because of rust (the inside of both MC's beyond this seal were perfect - no rust, no pitting, etc.). The first began leaking not too long after I got the car (1988-89) and lasted 13-14 years before leaking again. This time I put some silicone grease onto this seal in the hopes that the rust that caused the last two to leak will not occur again (if you try this, use only silicone grease and put it on the seal, do not put grease in the MC because the seal will push excess grease into the MC and into the brake fluid perhaps causing a clog and regular petroleum based grease will cause damage to the seal - it will swell).
So far everything looks good, but I replaced the MC only 10 days ago. I believe the rust may begin because I wash the area of the MC/booster from time to time and a bit of water got in there. To prevent water from entering this area I smeared a bit of silicone across the junction of the MC and booster and left an area on the bottom unsealed so if the MC leaks the fluid will come out there and not into the booster - and I will know there is a problem. However, the outside area on the MC where the leaking seal is located did not have any rust indicating that water was not contacting this area because it would rust. So, I'm a bit perplexed why the interior of the MC where the seal is located was rusted causing the fluid leak, but the inside of the MC just beyond the seal was perfect (no rust, no pitting, etc.) and the outside area where this seal is located did not have any rust.
Also, before installing both rebuilt MC's I took them apart to inspect everything and there was no rust anywhere, including where the seal that eventually leaked is located. I change the brake fluid every year to prevent rust/corrosion, so I'm a bit pissed that rust killed this MC. Also, when I removed the MC the o-ring seal for vacuum was good (you should have heard the inflow of air when I removed the MC) and the grooves for the o-ring did not have any rust in either the MC or booster. However, the area just outside of the o-ring grooves were rusted on both the MC and the booster housing.
Go figure!
!
Bleed the MC before installing it (if you replace the MC). I suggest the use of the Mighty Vac vacuum tool to bleed the brakes (about $30 from many auto parts stores - O'Reilly, NAPA, Pep Boys, etc.).
Good Luck!
Tom