I've got a 500ml plastic bottle of DOT4 brake fluid that weighs 400 grams that was opened 2 years ago. I know most people's knee jerk response says "throw it away because it absorbs moisture" as if it's some mystical substance from Lord of The Rings. One brake fluid to find all the water and bind it. But I'm not a sheep who follows mainstream advice blindly. I'm a scientist. So when I thought about it logically, this is what I figured (without even running the calculations)... but I've run some calculations below for the skeptical believers which has confirmed what I figured far beyond what I expected. I don't need to be super precise, so for the sake of simplicity I'm going to assume that there's 350ml of brake fluid and 150ml of air in the bottle, because only water is 1 gram per ml and obviously the bottle weighs a few grams, but whatever. At 25C and 100% humidity 1 liter of air will hold 0.017ml of water. But where I live the humidity is rarely above 20%. So let's be conservative and calculate using 40% humidity which gives 0.0068ml of water per liter. Now there's only about 150ml of air in the sealed bottle, so that means there was 0.00102ml of water in the air in the bottle when 150ml was poured out one time. That's 1.02 micro liters of water in the bottle. Now if we assume that 100% of the water from the air was absorbed into the brake fluid. That means in percentage terms, there is 0.000291429% of water in the 350ml. Not even 1000th of 1%. I assert that if that micro liter boils it's not going to make any measurable difference. Not even on a MotoGP racetrack with the most sensitive of equipment or laptime measurement. And I'm riding on the road which is a lot more relaxed than motoGP. So I call bullshit. Most likely this "common knowledge" is just propaganda spread by brake fluid companies. If you disagree, I look forward to your substantiation. Prove me wrong? And not with heresy he said, she said authoritarian (with conflict of interest) bullshit. Show me some calculations and facts? Other arguments I've heard Brake fluid absorbs water through the plastic bottle. I call bullshit, where's the proof that water goes through plastic? Have you noticed that water is sold in plastic bottles. I don't see any of it leaking out. So on a practical note. I intend to inspect the colour of the fluid. If it's clear I'm going to use it. And some people might mention that 350ml of brake fluid doesn't cost much etc. And no, it doesn't. But this is a matter of principle. What if I come up with a reason for you to pour out your drink every time there's 200ml left, just because it doesn't cost that much. Are you going to do it? What about right and wrong? Fact and fiction. Humans didn't evolve and advance from believing bullshit. We evolved because we practice the scientific method.
Actually I read on the mv forum earlier that sadly he tripped over an optimate cable last week and has only just been released from hospital. The front door keys under the plant pot if anyone wants to visit him ....I'm giving it a miss personally...
I only got half way through the original post and fell asleep with my face in a bowl of soup. When I awake I intend to give a thorough analysis of said post t'is a very deep coma like sleep though so don't expect it any time soon
Wurth brake fluid is £4.99 for 500ml from Sportsbikeshop. Motul RBF 600 Factory Line is £13.60. So if it lasts 2 years it would cost under £2.50 a year for the budget option, or £7.80 for a competition spec fluid. Why risk it for those prices?
with small amout of air in there I doubt if enough humiditiy was trapped in to cause any significant or noticeable moisture absorbtion. Most fluids are very pale yellow when new and become light urine colour after 2 years and darker still further on. I used a motul twice over 6 months (50% each time) and could not detect any color change or performance drop. Mind you I kept it in the airing cupboard before opening and after. But as barstw.. said getting new fluid is not that expensive
I agree with you @Unity. I didn't need the calcs. Sealed bottles don't absorb moisture. However can it degrade with age? I'd be using it, if only to be environmentally friendly. Unless it was originally bought on eBay.
Does brake fluid change colour with air/moisture? I thought it was heat that caused it, hence it going darker quicker on hydraulic clutches than brakes.
I bet a lot of brake systems ( car and bike ) have fluid that is over 5 years old with no issues. I would have no issues with putting it in mine
Original poster is from South Africa. You think more about these things in countries where the real £ cost us higher. But I didn't read it either!
I have been guilty of this but I have recently concluded that moisturised D) brake fluid can corrode the inner metal parts of the system like the bleed nipples. So from last year, I switched to 2 year policy even though I have not noticed any change in perfornance. More so on cars with ABS barst.. you may be right; also add sun light to the equation then it gets complicated. My observation were based on my car fluid and bike brake fluid that were > 5 years old
I've always thought this was due to the rubber seals wearing & the small particles causing the discolouration. More seals in the brake than clutch. The main reason for changing it is because the boiling point drops when it's diluted through hygroscopy. Fluid manufacturers quote a 'dry' (new) boiling point of around 230 Celsius & a 'wet' (saturated) boiling point of around 160 Celsius. The wet figure is after the reservoir (and its little vent hole) has been exposed to all of the air in the world, as opposed to a little bit of air in a sealed bottle. Most manufacturers say replace @ 2 years, but @Jez is right regarding many vehicles on the road having ancient fluid. This will be true despite some garages claiming to have replaced the fluid yet have only siphoned out the reservoir & topped up with new! This is probably/possibly because of issues with trying to undo bleed nipples that haven't been undone before (& therefore haven't had a bit of fluid up the threads to help). A skilled mechanic should be able to predict the likelihood of a bleed nipple breaking & advising the customer of the potential cost. BTW, if they're stuck, it helps to use a hexagon spanner to apply pressure & tap the bleed nipple lightly with a hammer.
