Mayf,
I looked into the question of hydraulic fluid compressibility a little and
found out there is general agreement that silicone brake fluids are two to
three times as compressible as DOT 3 or DOT 4. Here is a link to a very
informative article on the subject by Brian Smart, Service Technical, AP
Lockheed. I also found other generally corroborating references.
http://members.aol.com/tfrecord/page30.html
My original statement was "...all fluids are, for all practical purposes,
incompressible." First, to be accurate, I should have said liquids, not
fluids. Second, I should have qualified what practical purposes I was
referring to, which I intended to be limited to hydraulic systems. However,
upon further consideration of the facts, it looks like maybe I was wrong, at
least to some degree, for even this limited range. For some perspective on
this, take the case of water, for which I could readily find data. The
compressibility of water is such that if you, for example, pressurize it to
100 psi, it compresses about 0.03%. I don't know exactly what pressure you
produce in a brake system, but I would think a maximum on the order of 1000
psi. Assume we use water as the working fluid in the brake system. Then,
under hard braking with 1000 psi applied, the water will still only compress
by about 0.3%. My first impression is that this is totally negligible.
However, we must consider the total volume of fluid under pressure and the
bore and mechanical advantage of the brake cylinder in order to find out how
much pedal travel that translates into. I don't know the answer, but I can
imagine that it could wind up being a noticeable amount. I found the
compressibility of ether and it's almost six times as great as water. One
particular "liquid silicone", polydimethylsiloxane, has about 2.5 times the
compressibility of water.
In summary, it is clear after surveying the subject that experts who should
know seem to agree that the compressibility of the hydraulic fluid really is
a factor in pedal "feel", and that silicone fluids are two to three times as
compressible as DOT 3 or 4. I still remain of the opinion that if you change
brake fluid from DOT # or 4 to silicone and feel a noticeably spongy pedal,
the problem is most likely air in the system. I believe this because I have
used silicone and not found it excessively spongy when the system is
properly bled. Tom Hall and others have had the same experience. Also, some
sponginess is not necessarily a bad thing. For example, it can lessen the
tendency of an untrue rotor or out-of-round brake drum to lock up.
Bob
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