Mark,
I don't think anyone answered your question about DOT 4 versus silicone
brake fluid. DOT 3 & DOT 4 are both made from a Polyalkylene Glycol Ether
base. However, I don't know for sure that all DOT 4 fluids are compatible
with natural rubber, but Castrol/Girling DOT 4's definitely are. The
following link has the usual admonition about using anything but
Girling/Castrol brake fluid. DOT 4 is also supposed to have a higher minimum
boiling point than DOT 3 (401F vs. 446F). All DOT 5 fluids are silicone base
and have a minimum boiling point of 500F.
http://www.importautoberdoo.com/brake.html
Here's another link that discusses the pros and cons of PGE versus silicone
fluids. I've tried both. Personally, I prefer to use DOT 4 and replace it
once a year.
http://www.tirekingdom.com/purch/brkfluid.html
Bob
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