Purchased a 2012 IS 350C this year with 46,000 miles. Can find no evidence the brake fluid was ever changed. It tests positive for water. At this point should I leave it alone or pay to have it changed,

 

Definitely don’t “leave it alone” - why would you? This just guarantees problems sooner rather than later.

If you are concerned about water and are not worried there is air or contaminants in the system (which is usually the case), there is a very easy way to get the water out. Technically called “fractional dilution” but basically you swap the reservoir volume a couple times and you will get most all the water out. The reservoir volume mixes with the rest of the system after a bit of driving; not instantly.

- Siphon out the reservoir, refill.
- Drive a week
- Repeat, ~2x times.

If you have test strips or a meter, you can verify this. You’ll loose 1/3rd to 1/2 the water in the system with each drain/fill. Pro Tip: if you live in a wet climate and don't track the car, switch to DOT3 fluid. It is less hygroscopic than DOT4, and the boiling point you give up is pretty marginal, and irrelevant in street use.

 

If you are only able to do one thing consider turkey baster the reservoir as a brake fluid service. However, with the system not circulating fluid, I would feel more comfort in performing a complete system flush. Just remember whatever you do keep the fluid level the same at the reservoir afterwards, since pad lining wear has drawn fluid out of the chamber.

 

The reason I asked is when I searched online there seem to be some disagreement as to how important this is. As all the cars in this forum are over 10 years old, I was curious as to how many are actually having it changed regularly.

 

Brake systems fully mix the reservoir fluid with the fluid downstream of the master cylinder. Change the reservoir enough times and you (almost) fully change the fluid. Same as drain/fills on a transmission. Not as efficient, but very much easier.

 

I decided to have it changed. They drained it at all the wheels and forced the old out with the new. Had it done at a local tire shop that has been recommended by a few neighbors. Cost was $179.00 which included the hourly labor charge of $160.00. The good news is I was for the first time able to thoroughly look under my car. No sign of leaks and very little rust if any.