Hi Goldie,
Since you have a 67 BJ8, I must assume you have a Girling Mk 2B servo. Differing from my Girling Mk 2A servo that has its vacuum connection at the top of the piston canister, The 2B has it at the bottom. A while back, I had a leak in the servo's brake unit that leaked fluid into the canister and eventually filled the unit. The Mk 2A showed no indication of errant fluid and the fluid just seemed to disappear. This can not happen to the Mk 2B which, opposite to the Mk 2A, has its vacuum check valve on the bottom of the piston canister. I expect the disappearance of your fluid, if from the servo, would be sucked from the servo into the intake manifold and burned or at least expelled. Check your rear carburetor and rear plugs for any indication of excess oil deposits and/or burning.
One last suggestion, I also had the master leak in the cabin and, because it was not excessive, was absorbed into the padding under the rug (which was in close proximity). Again, it took me quite a while to find the leak as the loss was apparent at the reservoir but the cabin rug and brake plunger cover seemed dry (and again required getting under the dash to pull the rug apart).
Ray(64BJ8P1)