GraemeSmith wrote:A problem with older (time in service) Automotive fluid is that it has absorbed quite a bit of water into its molecular structure. It is thus prone to freezing at altitude.
Agreed.....most automotive brake fluids have a great affinity for water. I ain't buying the freezing at altitude, though. Unless -10F is different at 10,000ft than it is in a Michigan driveway in Feburary.
Automotive brake system reservoirs are not vented directly to the atmosphere, but have an accordion-like seal to take up space without allowing moisture laden air to contaminate the fluid. Given that some H2O is going to get into the system, regardless of the care taken, there must be some allowable percentage. That information is probably archived along with the Dead Sea Scrolls and Ten Commandments.
As a matter of fact, engine coolant's freezing point goes DOWN with the introduction of water. To a certain point. Does brake fluid possibly exhibit the same characteristic? Any articles I've read on moisture contaminated brake fluid revolves around corrosion. Never seen a reference to "frozen fluid".
Having said all that.........my Sonex supplied master cylinder had a sticker on it reading "DOT 5 fluid only".