Every manufacturer specifies flushing the brake fluid, ... And yes, no one does it. That's because the hydraulic systems cause so few problems, we ignore them. Every manufacturer specifies oil change intervals too, but I haven't changed the oil in my '88 Grand Caravan in over 12 years. That proves I'm ignorant, stupid, cheap, or I know something else. It does not prove the manufacturer's recommendations are wrong or frivolous.
Brake fluid boils at well over 400 degrees F, but it also loves water. It will absorb moisture out of the air if you leave the reservoir cap off. That's why all containers of brake fluid tell you to keep them tightly sealed. That moisture will also get in through leakage under the rubber bladder seal under the reservoir cap. Most experts also say moisture is going to sneak in through the rubber flex hoses because rubber is porous.
Regardless of how the moisture gets in there, it is going to lower the boiling point of the fluid to 212 degrees F. It is real common and easy for brake parts to get hotter than that. The water will vaporize, and since a gas can be compressed, you will develop one type of brake fade that results in a low and mushy brake pedal.
The second problem is that moisture promotes corrosion of metal parts in the hydraulic system. That's why we see rusted steel lines that are still coated in rust-proofing material. The lines rusted from the inside. To add validity to this story, all experienced brake system specialists know to never push a brake pedal more than half way to the floor when bleeding or adjusting the brakes. Crud and corrosion build up in the lower halves of the bores in the master cylinder where the pistons don't normally travel. That is not a problem yet on a car that's less than about one year old, but after that, running the brake pedal all the way to the floor runs the lip seals over that corrosion and rips them. That results in a slowly-sinking brake pedal but it might not show up for a few days. That is a common secondary problem after a driver is suddenly surprised by a ruptured rubber flex hose. Brake fluid does not corrode metal parts, so where did that corrosion in the master cylinder come from? Yup; from the moisture that Honda wants you to get out of there. Some manufacturers specify changing the brake fluid every two years.
Some manufacturers purposely specify real long maintenance intervals for "normal service" to make their cost of maintenance appear to be lower than that of their competitors, and therefore the cost of owning that car is lower. Ford used to specify oil changes at 7,500 miles for just that reason, but if you read what is allowed under "normal service", no one can meet those requirements. If you drive at city speeds or highway speeds, you fall under "severe duty". If you drive on dirt roads, that's "severe duty". Excessive short-trip driving and excessive highway-speed driving are "severe duty". We used to joke that if you drive too much after dark, or make too many right turns, you need to change the oil more often. What Ford and some other manufacturers call "severe duty" is really just normal driving.
Their comment, "no matter the mileage" is correct. Brake fluid doesn't go through the warm-up / cool-down cycles an engine does. It has no idea how far you've driven. The only variable is if any brake work was done that required bleeding the system. That would have exchanged some of the fluid, but not all of it.
I also have a '93 Dodge Dynasty that has never had the brake fluid flushed. This is a case of "do as I say, not as I do". The issue is not that the car has only 4,600 miles and has been in storage for 18 years. The issue is it's 21 years old and so is the brake fluid.
I don't know what they're recommending for the transmission. The standard used to be to replace the filter and the half of the fluid that would drain out with it every 36,000 miles, but that has changed for a lot of manufacturers. Transmissions have gotten much lighter to improve fuel mileage, and most are run now by computers that are very complicated and unreliable. There are a real lot of transmission problems now but not necessarily total failures, and some people believe flushing the fluid will overcome some of these problems. Normally, if a real problem develops, it's mechanical in nature and new fluid isn't going to fix that. If a problem hasn't developed yet, there's nothing for that new fluid to "fix" so there's no point in replacing it. You'll get as many opinions as people you ask if flushing has any benefits. Again, I'll defer to my rusty trusty Grand Caravan. It had the filter and fluid replaced only once at 84,000 miles only because a $3.50 side cover rusted out and the fluid fell out when I replaced it. Figured I might as well do the whole job right away. Today, this daily-driver has 263,000 miles, and I pull a tandem axle enclosed trailer twice a year that's bigger and heavier than the van. Other than that side cover and one filter, nothing has ever been done to it.
Obviously I'm not suggesting you ignore your maintenance like I have. This started out as a sort of experiment to show my students what could happen or what some engines are capable of. It has turned into a "why mess with what works". This is not neglect. This is abuse.
The jury is still out on whether flushing transmission fluid is a good value or has real benefits. I'm leaning strongly toward it's another gimmick to sell a product of questionable value, but I could be proven wrong someday. I AM in favor of replacing the filter and fluid regularly. Just like in engine oil and antifreeze, there are additives that wear out in transmission fluid. Replacing the fluid replenishes those additives, and getting the filter out of there removes the debris that got caught in it before it can make its way into someplace where it can tear a rubber seal or cause a shift valve to stick.
Sunday, October 26th, 2014 AT 10:51 PM