master cyl. losing fluid, no evidence of leak

[98 blackjet]

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.

[boschmann]

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.

[surferfletch]

The wheel cylinders are prone to leaking.