master cyl. losing fluid, no evidence of leak

I recently replaced the master cyl on my 98 jetta. worked fine for about a month, but this morning my pedal went to the floor and i almost t-boned another vehicle. I checked the resevoir , it was low so i topped it off. A few hours later it was down again. I checked every wheel and what brake lines i could see for leakage. Not a drop anywhere , no puddles, nothing. I'm baffled as to where the fluid is going. Could it be leaking into the vacuum canister? All advice would be appreciated.

If you keep putting it in the reservoir you will eventually find where it's going. You should back the master away from the booster to see if it's leaking there.

The wheel cylinders are prone to leaking.