On 18/05/16 21:03, polygonum wrote:
On 18/05/2016 20:45, Adrian wrote:
On Wed, 18 May 2016 20:36:04 +0100, Clive George wrote:

Neglecting changing DOT fluid is the single biggest reason for
replacing brake and clutch cylinders and calipers, not to mention the
risk of the fluid boiling.

Why does not changing the fluid make it more likely to boil? I'd
understand if you said that any water in the fluid could turn to
steam... Or if I were convinced that any water content would cause
depression of boiling point...

Not in the least arguing against changing the fluid.

DOT fluid is hygroscopic, ie absorbs water. The longer you leave it, the
more water there is in it. Changing it means you get back to fluid with
no water in it.

....and the boiling point of the water in the fluid is much lower than
the
boiling point of the fluid. Heat the fluid to 100degC or more, and the
water in the fluid boils, turning to steam. And steam is compressible.

The old DOT3 boils at 205degC. DOT4, the most common stuff today, boils
at 230degC. DOT5.1 boils at 260degC, as does the silicone DOT5, which
isn't miscible with the other three.

I am delighted to read about absorbing water, creation of steam,
changing the fluid so it is dry, and the actual boiling points. All
relevant and some mentioned in my question/response. But none of them
seem to support the claim I was questioning: that not changing the brake
fluid makes the brake fluid itself more likely to boil.


Because it absorbs water over time from the air (via the breather hole
in the cap for one).