Kendall---Here's one answer, per your request. I subjected MC seals to
DOT3 and a synthetic brake fluid for a six month test in sealed jars of
each type. The diameters of both seals swelled no more than .005 as best
as I could measure, due to their softness. This was compared to a new,
out of the box seal.
I've been using Valvoline's Synpower DOT4 brake for about two years now.
Too soon to tell if this is going to be any better, as the Castrol
usually gave about four or five years of service before seals needed
changed. About the only thing I've noticed so far is there's none of the
black swirly stuff hanging around in the reservoirs. (I think this was
Check out the new British Cars Forum: