[Roadsters] DOT5 (was: Clutch Question)

[dave]

As far as I know, DOT 3 and 4 are chemically similar, both absorb water, DOT
4 has a higher wet boiling point.  DOT 5 is silocone based, does not absorb
water, but any water in the system can collect at low points and cause
either local rusting, or at very high temperatures, vaporize and cause a
soft pedal from the "steam"
If I understand this correctly, an advantage to DOT 3 and 4 is that it can
absorb some water, and suspends it, preventing rust.  Once it is saturated,
then it can be a problem.  DOT 5 by comparison does not absorb water from
the air, so less gets in, but any that does get in can be a problem.

5.1 seems a good solution, non silicone, will absorb water, and a high wet
boiling point.

-----Original Message-----
From: Richard Schnoor [mailto:machworx at gmail.com]

Its been a while since I have worked on my roadsters, but I have some
questions: Forever DOT3 was the standard brake fluid which is hygroscopic
(absorbs water from high humidity air) which then ends up rusting your brake
calipers. Some one told me about DOT 4 which was silicone based & was not
hygroscopic so rusting wasn't a problem for a seldomly driven car. When I
changed over to "4" my brakes were extremely spongy & was accordingly was a
problem with this type of brake fluid. I even built a pressurized auto
bleeder so I could bleed my brakes alone but resulted in no reduction in
sponginess. So what is DOT 5 & 5.1? Do these new fluids address the
sponginess or rusting problem?



--
Richard Schnoor P.E.