87 SDL master cylinder fluid loss mystery

[clickclak]

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.

[Aquaticedge]

Check the rubber parts of the brake system. I'm not 100% sure but there may be a rubberized 'Tail' that goes from the hardline to the Caliper. Rubber weeps oil over time and might be the source of your leak. if you have access to a lift, get it up in the air and go over the components to be 100% sure none of them are weeping/leaking. or, you can possibly get a tracer type dye (if they make it) that illuminates under black/UV light if that runs through the system you'd be able to see where it leaks from that way too.

[Hit Man X]

Sucking back into the crankcase. Happened on my SD by way of vac pump.

New master and oil change cured my ails.