Moister contamination in brake fluid (or any hydraulic fluid) is most certainly not NOT a myth. You are wrong.
I agree that it is possible for it to get in, many years of seepage through the rubber lines, resulting in a small percentage of water that does not need removed nearly as often as every couple years, except if as already mentioned, there is some racing or other severe duty (like driving down long steep mountain roads).
Moister gets into the system every time the cap on the master cylinder is opened. it also gets in by going right through the rubber hoses. As moister accumulates in the brake fluid the boiling temp of the fluid is lowered. This is felt in brake fade as the fluid boils at a lower temp.
Why would anyone open their master cylinder cap unless it's already leaking and needs refilled, so the leaking component is replaced and the fluid bled after that repair? I sometimes check the level on mine but I use a flashlight if I can't see the fluid level without one, not taking the cap off.
It is true the boiling point is lowered, but this has never been a problem for me. If boiling point was so important then why isn't everyone rushing out to replace their DOT 3 fluid with DOT 5.1?
Most people do not replace their fluid every two years and do not have a problem from this. If you do find your specific vehicle and use results in brake fade from boiling fluid, then of course you should replace the fluid, but in that case it might be the pads instead, or that both need replaced, and possibly the rotors are glazed over too and so they also need replaced or lathed (if thick enough to allow it).
Most people simply do not face conditions severe enough to need this. Ever since the introduction of ABS, the brake systems have been more turned to increase stopping power instead of tuned to a range of human modulation to reduce lockups.
Also internal brake parts begin to rust. To eliminate this you can switch to silicon brake fluid, but this requires replacing all of the brake system components, because the silicon fluid is not compatible with regular brake fluid. I've restored/repaired many classic vehicles and what happens to the brake fluid over decades is quite disgusting. This is not only due to moister accumulation, but also to deterioration of rubber seals. It even seems to eventually crystallize if not maintained. After replacing all the brake system parts I use silicon brake fluid in my classic vehicles.
Over decades is the other extreme compared to every 2 years, and yes the seals will break down and produce black muck in the fluid. A small % of water present or missing doesn't change that and once the seals are shot they'll need replaced whether the fluid was changed every 2 years or not.
Like any fluid used in a vehicle brake fluid should be replaced periodically. Every 2-3 years (or 30,000 miles) is recommended. Running the same brake fluid in a vehicle for 20 years is not wise. You may get away with it, but your braking performance is compromised whether you realize it or not. Would you recommend not changing engine oil, trans fluid, gear oil or power steering fluid for 20 years?
Depends on the goal of the owner. Some will retire or sell a vehicle by then if it wasn't sent to a junkyard first for being in an accident. The vehicle is of very little value at that point anyway so how much work over how many years is it worth to preserve something worth less than $2K after that many years?
Engine oil is a different case because of cylinder blow-by that contaminates it. Trans, gear, or steering fluids, again this is taking opposite extremes. It would be excessive for me to replace those every two years too, except the tranny fluid if it were certain Hondas/etc known to have weak transmissions that need that.
Don't believe me? Do some research on the subject. Most manufactures (granted not all) recommend changing brake fluid every 2-3 years. Brake fluid is inexpensive and fairly easy to flush, as long as you don't get air in the ABS pump. Why not change it?
It is easy for someone to say it is safer to replace it than not, based on some hypothetical worst case scenario, having never driven the vehicle nor knowing whether there has been any braking problems from moisture.
I don't recall too many topics on this forum where someone had a brake problem and all they needed to do was flush the fluid?
I don't recall my Explorer owner's manual specifying replacing the fluid every two years. It states to check the fluid level and top it off annually. I checked the Ford Maintenance Guide too, and it has no mention of changing the brake fluid, only to inspect the brake components at certain intervals.
Why not change it? Because it doesn't need changed that often. I'm not suggesting that it will never need changed, rather that most people will get minimal benefit doing so before a repair requires it.
What I do recall is that the myth about changing the fluid this often was largely started by people affiliated with quicklube places that make a pretty penny upselling $200+ worth of services to people who came in for a $25 oil change.
Yes the boiling point lowers with more moisture in it, but that does not automatically mean the boiling point is reduced to a dangerous level. If it were, you better believe that Ford would have put that in the owners manual because widespread brake failure resulting from lack of changing the fluid every couple years would be a recall situation.
