Do you need brake-fluid replacement?

Brake fluid is hygroscopic. It pulls water from the atmosphere gradually over time.

The extra water lowers the boiling point of the resulting compound fluid. Boiled brake fluid is a reeaally bad thing-- the phase change to steam spikes the pressure which can unpredictably lock the brakes on one wheel or another. It takes quite a long time and some bad circumstances to have this failure. But it's good to avoid.

Additionally, the compromised fluid can promote (or at least not resist as well) corrosion damage inside those expensive brake booster valve bodies. This is a much more Prius-specific problem.

So read the maintenance manual again. It's in there for mine, and ours is about due.

 

Okay well that trips over one of my long-standing problems with Toyota's maintenance manuals.

They clearly say you should be inspecting that brake fluid's condition every 15,000 miles. But they don't tell you what the pass/fail criteria is.

You're not just going to look at it, judge the "condition" and shrug and walk away, are you? Well, hopefully not.

...but what exactly do they want you to do? They don't say. The implication is that if you don't know enough to judge the condition of brake fluid, you should take it to a pro. But it doesn't even spell that out.

That does not leave you off the hook.

You can buy testers and you can study the topic to your own satisfaction- but you should be aware of the general risk (steam flashover) and the specific risk (the Prius brake booster) and factor that into your interpretation of the maintenance guideline.

 

Well, they should be dipping one of these in there - before telling you need to change it.
Brake Fluid Tester Pen 5 LED Auto Oil Moisture Diagnostic Tool For DOT3 DOT4DOT5 | eBay
I think Honda or VW states 3 year replacement. I don't recall which one, just remember reading it somewhere.