Yes it is possible that it's going into the booster, a bad master cyl. can do this. Pull the vacuum line and examine it, it should be dry (no brake fluid). Since yours is a diesel, it may have a little residue from vacuum pump, I'm not sure about that, but I've never seen a diesel with out residue of some sort on everything under the hood. Be your own judge, try smelling the fluid/booster as brake fluid is rather distinctive. Good luck.