My dad's 87 SDL has been losing brake fluid for some time, about six months. Not a lot to be noticeable, but I see white smoke coming out the tailpipe. Initially I thought bad valve stems, and at 260K it made sense until I discovered the fluid loss in the reservoir.

After searching the forum for fluid loss I did the following:

Removed all four wheels and inspecting the calipers, as well as the rear brake lines under the subframe, and found no leaks. Pulled the MC and found no evidence of fluid in the booster, but went ahead and bought a new MC and will install tomorrow just to be safe.

I replaced all four brake lines 3 years ago, front rotors, replaced the seals in both front and rear calipers at the same time, new brake pads, and pressure bled the system.

Questions:

Since I can't find any fluid in the booster, where could it be leaking ?

Could the fluid be leaking slowly enough to be pulled back through the brake booster hose going to the vacuum pump, then burned in the engine, resulting in white smoke ?

Could the fluid be further inside the booster, behind the seal with the pin that aligns with the MC piston ?

Thanks in advance.