Master cylinder back pot empty

Either it is in the booster or in your floor board being soaked up by the carpet. Check the inside of your firewall.
That is the likley spot...if it did go into the carpet, the carpet should all come out and be thoroughly cleaned and all the fluid removed from everywhere. It is hard to tell how many floors have rotted out due this type of leak. If you are not aware, brake fluid absorbs water and will hold it against sheet metal.