I'm getting new pads & discs fitted to my 2015 X250 XFS this week - as far as I can tell from the service history the brake fluid hasn't been changed since 2019, 20,000 miles ago. Am I right in thinking it is overdue a change? What type of brake fluid is best to use, how much is needed and is there any special procedure to follow to change the fluid? I thought it makes sense to get the brake fluid and give it to the garage changing the pads & discs. Many thanks in advance.

 

Personally if I have a good pedal I leave brake fluid alone.

 

The fluid should be changed every couple of years. Brake fluid is hygroscopic- meaning it absorbs moisture from the atmosphere- and increased moisture content lowers the boiling point of the fluid. No problem will be apparent from pedal feel but in conditions where repeated hard braking- like driving down a mountain or similar- raises the temperature of the fluid it can exceed it's now-decreased boiling point and turn to vapour which has no hydraulic effect and brakes will stop working when you want them most (this is different to brake 'fade' which is due to the pads). I personally think the 2-yearly recommendation (miles are irrelevant) is overly cautious for most drivers but new pads & discs is as good a time as any.

 

Yes but it can't absorb moisture in a sealed system, so don't leave the cap off, that's what causes the problem. But I disagree the brakes will stop working, you will just get a soft pedal due to the air created.

 

Yes realistically it's never likely to be a problem in a modern vehicle, but like many things the sealed system is meant to be an added protection rather than replacing other precautions like keeping the fluid reasonably fresh. Theoretically if enough fluid is boiled to vapour, it can take more than the full travel of the pedal to compress the vapour so no actual braking results- I saw examples of this in training films over 40 years ago and the films were probably 20 years old then. I can't imagine what you would have to be doing for this to happen in a modern car.. But I'd be changing the fluid with new pads & rotors.

 

I realize that the brake system is a closed system but moisture does manage to find its way into the brake lines. I believe this is what causes corrosion of the brake calipers, which have to then be rebuilt -- an expensive proposition. I've always changed my brake fluid in accordance with manufacturer's recommendations and never had to have my brake calipers rebuilt. I know many people take the view that changing the brake fluid is unnecessary because the brakes work just fine and then they face a big bill when the calipers have to be rebuilt.

The one time I didn't follow manufacturer's recommendations regarding brake fluid changes because my mileage was so low, I finally changed my brake fluid myself and you could see the difference in the color of the old brake fluid vs. the new brake fluid when you put each in glass jar. The new brake fluid was a nice yellow-gold color and the old fluid was a putrid greenish yellow color. And the car had fewer than 5,000 miles on it. Apparently, the brake lines picked up some dirt along with the moisture.