Duncan,
This is starting to sound like a thread that ran a couple of years back on
the List. First, I have had the same problem you describe with DOT 5, but it
is not the silicone fluid per se that is compressible. In fact, all fluids
are, for all practical purposes, incompressible. It is, however, a lot
harder to get all the air out of the system with silicone than DOT 3 and 4
fluids - probably a viscosity and/or surface tension difference. Carefully
and gently pouring the brake fluid into the reservoir to avoid creating
bubbles and waiting a few seconds for them to rise to the surface before
bleeding helps.
The issue of moisture is more complicated. Silicone fluid has no moisture
affinity, or "activity" as Castrol's LMA states. Thus, if the system is
perfectly dry and you add silicone and store the car, it will probably stay
dry. But how do make sure it's dry? With silicone fluid, if there is
moisture there, it will collect at the low spots and cause corrosion. With
DOT 4 at least the moisture is evenly distributed and will be removed when
you replace the fluid. On the other hand, even a good flushing with silicone
will probably leave some water in the system.
Bob
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