Sources of water in brake fluid

[9xxNick]

I need to understand if there's any way for water to enter into the brake fluid of a road car other than through absorption in the reservoir.

I don't believe there is, but would be interested to understand if there is any other mode by which the fluid can become contaminated by water, other than if already-contaminated brake fluid's been added to the reservoir.

[9xxNick]

That's what I thought. I'm faced with a situation where it should have been completely changed about 22 months ago, but is showing as >4% water, which makes me very suspicious that it wasn't changed when it was supposed to have been.

[9xxNick]

To answer the questions above:

Supposed to have been changed: Yes, because the garage charged for it and I've previously found them to be fairly good as garages go, which is why I haven't done a positive check on that aspect of their work before.

Has it got hot: I don't think so - it's my wife's car and she's not hard on it, and no one other than her and I have driven it.

Most likely changed the fluid in the reservoir and nothing else: Possibly, but even then I'd expect it to be below 4% water after less than two years.

Garage is supposed to be coming back to me this afternoon.

[9xxNick]

One of these Brake Fluid Tester

I don't have a lot of confidence that it's very accurate, but I've done a kind of baseline against fresh fluid, older fluid and a couple of other cars, and they all come up as expected with very low readings (<1%) so it seems likely the car really does have excess water in the fluid.

[9xxNick]

I may have to do a quick check against new fluid which I've added a known amount of water to to see if the results look consistent with the percentage added. As said above though, the basic check against new fluid and cars with known good fluid checks out as <1%, with the suspect vehicle showing as >4%.

[9xxNick]

Yup, I'm aware of that and it's the reason why it needs to be changed periodically. My problem is that I appear to have very water-contaminated brake fluid even though it was supposed to have been changed less than two years ago.

[9xxNick]

I believe I'm fairly familiar with how this works, but there's no harm in checking, since I'm not a mechanic by trade, just an interested amateur.

Measurement came from a device similar to the Brake Fluid Tester I linked earlier in the thread.

[9xxNick]

That's interesting. There are quite a lot of graphs that seem to be plotting the exact same data (DOT3 fluid with the same contamination levels over time) so it's possible that the source data is the same for all of them.

It's still odd though that every car manufacturer I've come across where I can recall the detail seems to think brake fluid changes every two years is fine, which would seem to contradict the data in the graph.

[9xxNick]

I have tested it on other samples, as I mentioned earlier on. I get that it's rarely a problem, but if I'm paying for it to be done I expect it to be done.

[9xxNick]

Agreed, air has to get into the reservoir to replace the fluid that's taken into the brake circuit as the pads wear, but this is factored into the replacement time.

Anyway,I think we've done this one to death now, so thanks for the inputs.