Charles---DOT 3 and 4 are compatible, so nothing needs to be done if you
mix the two. You can pump the "old" fluid from each wheel cylinder bleed
nipple till you get a few ounces out if you wish, or let attrition take
its course over time when the system will be all what ever's put into
the master cylinder.
Much has been said about how DOT 3 is hard on cylinder seals and such.
Two weeks ago I put a (measured O.D.) seal into each of three bottles
containing the popular types, DOT's 3, 4, and 5. I'll check them in a
few months to see which have "grown." There's a new? fluid on the market
made by Valvoline, called Synfluid. It's also compatible with the glycol
based 3&4.
I'll give a report on this when I find out something. It's in my clutch
system, as of a couple weeks ago.
Dick T.
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