View my fuel log 2015 Mirage ES 1.2 manual: 51.9 mpg (US) ... 22.1 km/L ... 4.5 L/100 km ... 62.3 mpg (Imp)
Brake fluid is, indeed, hygroscopic. That means it absorbs water. (unless it's Dot 5 silicone, which doesn't... but, also doesn't work quite as well, is not necessarily compatible with other fluids, and is more expensive) Which is why you should never open your brake system when the humidity is high or when it's raining if you can avoid it. If you have any doubt as to how powerfully brake fluid likes water, leave some on your hands for a while. It will suck the moisture right out of your skin and leave your hands dry for days! (btw, never TASTE brake fluid... trust me on that one)
Brake fluid should be changed every 2-3 years to maintain full performance, more on that in a bit. An awful lot of people never change their brake fluid and will let it go for 8-10 years or more... however long they happen to own the car. Brake fluid tends to get darker as it ages. If you've got brown brake fluid, change it! It should be some shade of pale yellow, and most of it starts out almost clear.
What's the problem with "wet" brake fluid? Have a look at your brake fluid container. If it's worth using, it's going to have two specifications listed. "Dry boiling point" and "Wet boiling point". The Dry boiling point is the temperature at which that fluid will boil if you crack open a fresh can and flush your entire braking system with it. At that moment, it's got its maximum boiling point. As the fluid absorbs moisture from the air, it's boiling point gets LOWER, and the Wet boiling point is generally what you'd expect at about 2 years. Beyond that, it continues to absorb moisture and goes lower still.
The official boiling point spec for DOT 4 brake fluid is 446 F Dry and 311 F Wet. Notice the BIG difference between those two temperatures? You'll notice it if you ever get your brakes hotter than the boiling point of your fluid, I promise! Typical OE brake pads work in a temperature range up to 600 degrees. So, you could have almost that much heat in your pads after one or two good hard stops. After that, if you continue using the brakes, more and more of that heat transfers into your brake fluid. 600 degree brake pads... 300 degree fluid? See where this is going?
The average driver might get away with letting their brake fluid go for 8 years. The boiling point of their brake fluid will be well into the 200's. They never use their brakes hard enough to get them that hot, so it works. But, with a boiling point that low, all it takes is one good hard stop from highway speed and the fluid will instantly boil. No, not all of it. Just the little bit that's right there in the brake caliper. It boils and creates an air bubble, and THAT is the problem.
That air bubble doesn't go away. It stays in the system until you bleed the system and remove it. Hydraulic fluid does not compress. It is designed to transmit all of your pedal effort into movement at the brake. Air DOES compress! Air bubbles in your braking system at a minimum will make your pedal feel spongy and soft. At their worst, you got nothin'! The air absorbs all of your pedal effort, and none of it gets to the brakes.
Pro tip: If you ever feel a soft/spongy brake pedal (boiled brake fluid), that could lead to "fluid fade"... and you're screwed. It's not going to come back to you. You'd better be looking for plan B (Downshift! Does the hand brake still work?). If you feel a HARD brake pedal. You're pressing on the pedal, it's not spongy, it's quite firm... but, the brakes aren't working as effectively as they should... that is overheated brake pads or "pad fade". Pad fade WILL come back (if you don't completely cook the brakes), when you allow the brake pads to cool back down into their operating range. Knowing little things like that could save you on a race track, or on a mountain road. Pay attention to your brake feel!
Do you need "better" brake fluid? Meh. I'm of the opinion that "if it ain't broke, don't fix it". If you do a lot of spirited mountain road driving, or if you're getting into track events, it might be good insurance. (note that you DON'T need it for autocross because the runs are short with cool-down time between them, and you shouldn't be using your brakes all that much) But, if you've never experienced a spongy brake pedal due to boiled brake fluid, then you should be fine with FRESH standard DOT 4.
Obviously, a good "hypermiler" isn't likely to ever get the brakes hot, and won't need to worry much about brake fluid.
Just as an example, a good, inexpensive, "better" brake fluid would be ATE Type 200. It's dry boiling point is 536 (up from the standard of 446) and its wet boiling point is 374 (up from 311). There are more exotic ($$$) fluids that can get your dry boiling point up to 600 or so. Great stuff if you need it! But, you shouldn't be needing any of that for a street car. I just use Prestone Synthetic DOT 4, myself. (510/311, and I flush it every couple years, or anytime I'm headed to a track)
Sorry for the ramble. Brakes are kind of important, this is good stuff to know!
Simplify and add lightness.
I change my brake fluid only when I need to open the system to do something such as replace a caliper. I am not looking forward to the day one of my calipers gets stuck or starts leaking. Fortunately, brake calipers and repair kits are available on Rock Auto.
Changed the fluid today as brakes felt definitely spongy and it was long due anyways. Started with passenger rear and the first ounces of fluid concentrated a faint brown color in the bottom that transitioned to almost totally clear to the top 2/3 of the receptacle. I take that the fluid had broken down in that line.
Last edited by R01k; 09-30-2018 at 08:51 AM.
View my fuel log 2014 Mirage ES 1.2 automatic: 37.6 mpg (US) ... 16.0 km/L ... 6.3 L/100 km ... 45.2 mpg (Imp)
How much of the 32 oz bottle is used in a complete bleeding and refill? I just changed the brake fluid on my motorcycle and have 80% of the bottle still left. I want to know if this will be enough to do the Mirage?
Last edited by fc321; 10-09-2019 at 06:41 PM.
2015 Mirage DE 5 speed Manual (no bluetooth) nickname: "Agile Grey Rat"
View my fuel log 2015 Mirage DE 1.2 manual: 44.9 mpg (US) ... 19.1 km/L ... 5.2 L/100 km ... 53.9 mpg (Imp)