It is leaking somewhere....best guess, if you cannot see anything external(calipers, lines, etc), is the master itself, as has been mentioned. Take the two nuts loose that hold the master to the booster, and pull it out a little. If fluid runs out, or it is obviously wet in between the booster and master, you've found the leak. The master will need repair/replacement. Depending on how much fluid made it into the booster, it may also need to be replaced.
When you found the rear of the master dry/empty, and added fluid, then pumped the pedal, the level lowered because it was filling in the empty areas of the master and lines that had air. If the level went low that quick with a leak, you would certainly have seen where the leak is.
In some cases, the piston seals in calipers can leak, and not show moisture, if the dust seals on the pistons are in good shape..the dust seals will hold the fluid in..until they get so full they pop, and fluid runs everywhere.
______________
Joel Adams
C3VR Lifetime Member #56 
My Link
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
