Originally Posted By: emmett442
I still think this is all a myth
Where is the "dirt" and "junk" in the fluid coming from?........It's a sealed system.......................................................... Every time I check or replace the brakes, I remove the bleeder, anti sneeze, and reinstall.
Maybe someone is putting anti-seize in it? Y'know, oil-based, rubber incompatible stuff?
That (fairly slim admittedly) possibility aside, if it was a completely sealed system, you'd never have to change your brake fluid.
You do.
So it isn't.
Given that it isn't, its a fair bet that it isn't uniformly permeable, so there may be local concentrations of water. I've seen it stated that water gets in via hose permeability. If that's true, it perhaps also gets in through the piston seal. This would tend to produce a higher concentration of water in and behind the pistons.
In addition, there is movement and friction in the pistons (and master cylinder) which will tend to put debris from seal and bore wear into the fluid.
I've never seen an analysis of the distribution of water and debris in the system, but anecdotally the first bit of bleed tends to be darker, which tends to confirm expectation.
So if its a myth, its at least an unusually reasonable one, as myths go.