How much does brake fluid circulate around the braking system?

IME, stays where it is.

No. The primary contaminant is water adsorbed from the atmosphere. This is heavier than the fluid, so sinks to the lowest point. This is usually the rear wheel cylinders, and by a cruel twist of fate (!), these are the components most at risk from corrosion damage. You can see this if you bleed the rear brakes; it's only the first few pushes of the brake pedal that produces dirty fluid.

Chris

Water actually gets in from both ends - the M/Cyl and w/cyl - you will find that the m/cyl will also corrode. calipers will rust up as well but not as accelerated as the w/cyl. To keep your system in good order fluid should be changed (flushed out) on a yearly basis - probably something that is far too ofter neglected.

Even BMW only recommended a 2 yearly change.

I'd be interested to understand how water enters the system from the wheel cylinders, but fluid doesn't get out.

Even Ford only recommend a 2 yearly change. ;-)

Chris

Two years now may be correct but it was less, may have something to do with the properties of the fluid or spare part sales. The MGB manual says every 18 mths or 24000mls which ever comes first. I do mine every

makes you wonder! how it gets passed the one way seals, I think its the viscosity difference.

Its something less than a couple of percent water is the max in fluid.

In the good old days when they did good bogey jobs I remember brake fluid being boiled to remove the water and then reused. But wheel cylinders were made of better material and just a hone and new seals fixed the brakes. Now its an assembly replacement.

They did more maintenance back then when you could maintain your own car.

I thought the moisture only gets in from the reservoir as it's level goes up and down as you brake and the vent in the reservoir cap allows air in and out. The water is heavier than the fluid and sinks to the lowest point, the wheel cylinders, and rusts them that way.

Diffusion through the rubber seals. The water molecule is much smaller than they hydraulic fluid one.

The seals are protected by rubber boots, are kept mostly dry and sometimes warm, and the system pressure WRT atmosphere is either positive or neutral.

I'd take a lot of convincing that more than a trivial amount of water entered the braking system via the wheel cylinders!

Chris

Chris Whelan was thinking very hard :

Me too!

So far as I am concerned, the only mechanism for absorbing moisture in the fluid, is from the air/fluid interface in the master cylinder. Then I suppose that slightly heavier that pure fluid, fluid containing some moisture might make its way eventually down to the wheel cylinders.

Osmosis through the flexible pipes.

I don't pretend to know how water permeates into the hydraulic system but the water contaminated fluid from the master cylinder would surely have to go back uphill (from the underside of the floor) to get to the rear wheel cylinders/calipers (which are always mounted towards the top of the axle)?

Biggles

Bullshit.

I don't think the water needs to accumulate there, it's there that's made of cast iron so it'll discolour the fluid much sooner.

Perhaps it does all get in at the master cylinder, and gravity takes the denser wet fluid to the "low point", but then diffusion will take water uphill to the rear slaves. One of those things which gets more complicated when you look at it closer.

Certainly true that in neglected vehicles it's the rear slaves which suffer. I should know.