larry resnick

Hi Engifineer,
Air gets into the brake system from several paths. When the brake pads wear and the pistons move out to accomodate this wear, the fluid level declines, and any air that enters above the fluid in the master cylinder brings moisture with it. Next, when the pistons move out to engage the brake pads every time the brakes are used, they expose the cylinders they are in with a thin film of brake fluid, so when the pistons move back in the films of brake fluid that has been exposed to the air also has moisture in it and this fluid is now inside the cylinders behind the pistons. It's not much, but it is accumulative. That is why you see fluid that has darkened a long way from the master cylinder.

It is not so much air that is the problem, but moisture which is in the air, which is absorbed by the brake fluid, both lowering its boiling point and using the rust inhibitors that new fluid contains. As the rust inhibitors are depleted, rust then forms inside the system, which is what darkens the original brake fluid. So those are the two reasons to change the fluid.

As an example, I had a BMW E30 M3 which I tracked occasionally and bled the brakes every two years or more frequently, and for 30 years had no brake problems.
Larry

HarryD

FYI the clutch fluid cycles, perhaps because the system is quite small and compact.

But your break fluid does not cycle completely and/or fast enough.

E.g. your fluid can be kept fresh up top in the MC, but all the way down in your brake lines at the bottom of your system, your fluid will still be the old, dirty, murky stuff. Every time you press your brakes, the fluid isn't freely moving and cycling through the system, so turkey basting your brake MC is not enough to keep the entire system clean.

Bleeding the brakes is required to get that fluid from the MC flushed clean through the system.