Re: A similar chemical with heavier viscosity than DOT 3 brake fluid?

Dear George,

It isn't the oil itself in the DEXRON that ate your rubber gaskets, it was 
a detergent additive that did.  The oil itself is just some hydrocarbon 
chains that would be pretty innocuous to the rubber.  But since the DEXRON 
is meant to go in transmissions, it contains some additives that help break 
up deposits on the metal surfaces.  This is what ate your rubber.

So, if you want to use a fluid in your brake system, you can use almost any 
oil you want so long as it is detergent free (or nearly so).  You could 
even use vegetable oil that you get at the grocery (although I can't vouch 
for its longevity or smell after extended use!)  So what should you use?

Jint Nijman gives a recommendation for brake fluid replacements for old 
Citroen cars (at least that is what I deduced from the following page) 
http://ourworld.compuserve.com/homepages/Jint_Nijman/c
itroen/gen/Hy
draulic_Fluids_1.html
He mentions using DEXRON type A as a replacement for worn out brake systems 
as it is almost 3 times as viscous as LHM, a mineral based oil (likely 
similar to DOT 3).  I couldn't find an example of DEXRON A but I bet that 
it is a non-detergent form.  If you can't find that, you may want to look 
at some non-detergent hydraulic oils or possibly higher viscosity mineral 
or synthetic fluids (such as silicone based DOT 5).  A nice explanation of 
the DOT #'s is given here: http://www.vtr.org/mai
ntain/brake-fluids.html

You may need some assistance from someone who knows lube oils to choose the 
right viscosity for your application.  There are a number of different 
methods and measures for viscosity and I was unable to crack the code in my 
limited searching.  A quick reference chart comparing the major 
measurements is found at http://www.wostinson.com/vchart
.html

If you would like to learn more about viscosity, lubrication, and most 
anything else having to do with hydraulics and lubrication, Lubrizol has an 
excellent online reference library at: http://
www.lubrizol.com/referencelibrary/lubtheory/index.htm
This may be a little technical heavy but it may answer many questions if 
they come up.

I hope this helps a little.  The truth is a representative from some of the 
lubrication manufacturers may be able to help you a lot more, but at least 
you know what questions to ask (mainly, does it contain detergents or other 
additives that will eat rubber!)  Best of luck.
BK

P.S.  I take every answer seriously!  (Well, mostly).