I rebuilt my brake booster servo about 25 years ago and I have been running Silicone brake fluid during all this time. It has performed normally during that time. I used silicone (DOT 5) because the car has long period of inactivity and it has been my belief that silicone will not absorb moisture and therefore there will not be corrosion in the internal components. That does appear to be true now that I can see the results of a long time.
However, there has always been the argument regarding the silicone compatibility with the seal materials.
Recently I noticed a great deal of smoke emanating from the back of my car when I pressed the brake pedal. Then the pedal wend soft and I had no brakes. Fortunately I was in my driveway by that time.
The only place where there is a boundary between the brake fluid and the manifold vacuum is the seal in the servo where the actuator rod from the vacuum piston enters the boost cylinder.
Therefore, I bought a rebuild kit and proceeded to dismantle the servo.
When I removed the back plate from the vacuum piston chamber, it was full of brake fluid.
When I got to the part where I had to extract the old rod seal, I expected to see it deteriorated unable to seal. To my surprise, it looks fine. It fits the actuator rod with a reasonable degree of tightness and the rubber appears healthy. I don’t see any corrosion or defects on the actuator rod, so it is hard to see how the brake fluid leaked.
Am I missing something with regard to other leakage paths?
Mystery Solved… There is another leakage path for fluid to enter the vacuum side of the servo. That is by way of the “Air Valve Piston”. The rubber piston in my servo measures 0.520" diameter. The side walls look cylindrical. The replacement piston seal that came in the kit I bought is 0.530" diameter and the sides are clearly tapered outward. The inside of the air valve diaphragm chamber was wet with brake fluid.
This is a catastrophic failure mode as you would lose your brake fluid resulting in brake failure. Of course the fluid would have to make it through the engine causing great clouds of smoke.
Good you solved the Mystery , I rebuilt mine about 36 years ago , about 32 of them years it was in a box , still working well thou !
I rebuilt all the brake system , got new pipes and packed it away , in a box , most is on the car now , pictures where taken in the seller in our old house , car was in a lock up 2 miles away
I have rebuild mine years ago with Dot-5 and perfect…
However I did the same for the two (leather ) ones on my Gordon Keeble and the valve system didn’t like the Dot 5. Had to go back to normal brake fluid.Valve block looks the same to me as the later ones though.