Quote:
Originally Posted by jtur88
One critical question: Does the driver get any detectable indicator that brake fluid needs to be changed, or does it work perfectly one day and not at all the next? That would be an important factor in the "excusability" of not changing it at an appropriate interval.
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Short answer: No.
Long Answer....read below
What would happen is that over time, the fluid will absorb moisture into the system slowly dropping the boiling point of the brake fluid from it's DRY spec, to the bottom WET spec limit. Might take a year in a humid environment, or 5 years in a desert. Who knows.
But lets assume a fully, moisture laden brake fluid fill. So now instead of the fluid boiling at, say, 500 degrees F, the water laden fluid now boils at a lower temp of 350 degrees F. 350 is still plenty hot. I've driven my cars aggressively and noted brake temps of 200-250 degrees F just from average 50-0MPH semi-aggressive stops. So not hot enough to boil the fluid.
However, all you need is that one big panic stop. F-150 towing a boat or something like that. Get those brakes nice and hot and really build up some temp in the calipers and once you exceed that wet boiling point temp the fluid basically boils and creates vapor. Since vapor is not compressible, your pedal pretty much sinks to the floor and you experience probably one of the worst feelings ever in the pit of your stomach.
That's worse case scenario. usually what happens is the temps in the caliper build and slowly boil off small pockets of moisture not absorbed into the saturated brake fluid. While 350 is a pretty high temp and rarely exceeded by your average grocery getter going to the coffee shop on sunday, the boiling temp of water is 212, which is low enough to boil off here and there. It's usually small amounts, and over time, your pedal feel becomes spongy as the vapor in the system tends to accumulate. That's why many people who do regular bleeding comment on how pedal firmness increases after such an act.
So, while there is no definitive way to detect if the fluid is old and needs to be changed, some more astute drivers may notice the pedal feeling a bit spongy over time. However, I would be 99% of all drivers have no idea what they are feeling and just chalk it up to "that's just how the brakes feel" Usually on my daily driver I try to do it yearly and always notice an improvement in pedal feel. It feels firmer and more responsive.
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