Brake Fluid Hydrodynamics Question

[coppernorks]

Does the brake fluid in the fluid reservoir have the same level
of contamination as the brake fluid pushing the pistons in the
calipers, or does the fluid circulate round the system ?

or another way, is taking a reading for water contamination from the reservoir
a good indication of the state of fluid of the entire system ?

[GreenV8S]

The brake fluid doesn't mix much so the condition of fluid in the reservoir isn't representative of the condition in the caliper - which is what matters.

[stevieturbo]

coppernorks said:

Does the brake fluid in the fluid reservoir have the same level
of contamination as the brake fluid pushing the pistons in the
calipers, or does the fluid circulate round the system ?

or another way, is taking a reading for water contamination from the reservoir
a good indication of the state of fluid of the entire system ?

Brake fluid is supposed to be a service item around 5 years I think....

Does anyone ever change it ? very rarely.

But no it doesn't circulate, and no the reservoir is only an indication of fluid in the reservoir. Really if it is a concern and has never been changed...change it.
Not that it will likely perform any different, but it may give a little peace of mind.