Age of Brake Fluid
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Brake fluid is hygroscopic, so it holds moisture(time). It can also get dirty from bits from inside of the hoses and other components like the calipers (miles). Many people go their whole lives without ever doing it. I do it every 2-3 years in the cars and annually on the bikes. You have to get all of the old stuff out for it to matter. It is a very simple job made much harder by the formal instructions. lol
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Use a brake fluid tester about $20 my car will be 5 years and the fluid still in the green zone on the tester
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the reservoir.
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As mentioned before, the fluid slowly pulls water from the air.
I'm drawing a blank on the exact physics (flashes to steam under pressure making it spongy?) and exponentially degrades the chemistry of the fluid.
An extreme example is a motorcycle with 40yr old fluid is basically grease and the system is trash.
20yr fluid in a vehicle is maybe jello
For me, I just suck the fluid out of the reservoir then bleed the brakes to purge the old fluid 4ish years more or less, like during a brake job.
The cost of the fluid is relatively low and it's a low risk thing to do.
But that's me... At the shop they have seen brake fluid look like garbage (dark and jelly floating around) on a 3 year car ... Then see much much older that is OK.
There are variables involved i.e. climate and driving habits and engine compartment temps, and how long it stays warm each run.
IMO there is not a one-size-fits-all interval on brake fluid.
I'm drawing a blank on the exact physics (flashes to steam under pressure making it spongy?) and exponentially degrades the chemistry of the fluid.
An extreme example is a motorcycle with 40yr old fluid is basically grease and the system is trash.
20yr fluid in a vehicle is maybe jello
For me, I just suck the fluid out of the reservoir then bleed the brakes to purge the old fluid 4ish years more or less, like during a brake job.
The cost of the fluid is relatively low and it's a low risk thing to do.
But that's me... At the shop they have seen brake fluid look like garbage (dark and jelly floating around) on a 3 year car ... Then see much much older that is OK.
There are variables involved i.e. climate and driving habits and engine compartment temps, and how long it stays warm each run.
IMO there is not a one-size-fits-all interval on brake fluid.