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Anyone used/using something like this to check water content of DOT fluid…
Wondering if it’s any use or whether it will just result in me binning a load of perfectly good fluid…
As I understand it, brake fluid doesn't keep absorbing more and more water but actually reaches a saturation point. If that saturation point doesn't cause the boiling point to go low enough to cause a problem, you don't need to worry.
I use Motul RBF600 which actually has a wet boiling point higher than most dry 5.1 brake fluids. I'll change it no later than every two years and it's never been an issue.
I don't bother with any pen testers.
As I understand it, brake fluid doesn't keep absorbing more and more water but actually reaches a saturation point. If that saturation point doesn't cause the boiling point to go low enough to cause a problem, you don't need to worry.
I use Motul RBF600 which actually has a wet boiling point higher than most dry 5.1 brake fluids. I'll change it no later than every two years and it's never been an issue.
I don't bother with any pen testers.
+1 'good' brake fluid isn't always DOT5.1. Just buy decent fluid (it's hardly like bike brakes use much of it so splash out) and bleed it if/when it needs it. Although your statement about the relative wet / dry boiling points is off. 5.1 had higher standards (because it's newer) but also changed some fairly irrelevant things for bike use like the viscosity to help with ABS / ESP pumps. So 'good' DOT4 sometimes falls short of meeting some inconsequential spec ion DOT5.1. In RBF-600 it's the viscosity at -40C which isn't something most of us will ever test!
azupim01.motul.com/media/motulData/DO/base/RBF_600_FACTORY_LINE_en_FR_motul_48100_20220113.pdf
Most of the water will be getting in through the caliper seals as they actually move unlike car or motorbike brakes where the pads are essentially in contact with the rotor full time so testing the fluid in the master cylinder wouldn't give you the worst reading anyway. Unless you're doing huge mileages in wet conditions I'd be surprised if annual bleed isn't more than enough for most people.
It won't absorb a meaningful amount sat in a closed bottle on the shelf.
Same applies to my cars, I trust myself more to change the fluid every couple of year than to wait for kwickfit to upsell it to me with their magic tester.
Sorry yes, Motul RBF600 dry 312°C wet 205°C. Minimum spec for 5.1 dry is 260°C
Is this tester for a car or a bicycle? If it's a bike I'd just change the fluid every 12/24 months when you feel the brakes getting crap, as there's bevery little fluid in the system, compared to a car.
Same for a care TBH. For the sake of £50/£60 every two years I just get it done at that interval.
I use Motul RBF600 which actually has a wet boiling point higher than most dry 5.1 brake fluids. I'll change it no later than every two years and it's never been an issue.
Yep, I put this in the car after a messy trackday on some fresh but basic 5.1, it made a huge difference. It's not even all that expensive so I just started putting it in everything else. According to the internet it soaks up water faster than a more basic dot 4 but I'm not sure that's a real thing (tbh I think the limiting factor has to be the availability of water to soak up, or lack of?) Anyway, never found any downsides in the real world.
I was thinking it might be useful to test fluid that’s been sat on the shelf before using it rather than deciding when to change it.
Fluid in a closed bottle should be fine. The "wet" test is 3% water, not a negligible amount. Unless it's been left with the lid off for months then it'll be fine, if you're worried, put the bottle in a ziplock bag as well. Even googling throws up a few people who've bought similar pens dipping them in old bottles of fluid that have been stored for months/years and only lighting up 1 LED.
n.b. they measure conductivity not boiling point, so a 'basic' fluid that passes could have the same boiling point as a 'racing' fluid that fails.
If you really want to worry and go down a rabbit hole, buy some Castrol React SRF, it's £50-£60/litre and doesn't even meet the full DOT4 spec (but has a wet boiling point of 270C, higher than the Dry boiling point, 260C, in the DOT5.1 spec). I'm sometimes tempted but I don't think I know anyone else geeky enough to want to split it into smaller bottles! Whereas RBF-660 is half the price per liter and also comes in smaller bottles. And TBH when we're talking about getting the brake fluid to ~300C, mountain bike pad compounds simply won't be designed to get to the temperatures that could conduct that much heat through the piston (think about track car/bike rotors glowing red and sparks flying off the pads, just doesn't happen on a MTB). The more likely mechanism for vapor lock on MTB's is a poor bleed trapping some air in the caliper/hose that then expands enough to lock the brakes. Or people overfilling their brakes to try and reduce the free stroke meaning there is nowhere for the fluid to expand (it still expands as it warms up just like any material).
Fluid in a closed bottle should be fine. The "wet" test is 3% water, not a negligible amount. Unless it's been left with the lid off for months then it'll be fine, if you're worried, put the bottle in a ziplock bag as well. Even googling throws up a few people who've bought similar pens dipping them in old bottles of fluid that have been stored for months/years and only lighting up 1 LED.
n.b. they measure conductivity not boiling point, so a 'basic' fluid that passes could have the same boiling point as a 'racing' fluid that fails.
Old cars tended to have vented reservoirs, they were just a steel pot with a breather hole. Modern cars (and all bikes that I've ever worked on apart from the very early brakes) have bellows under the cap to stop air getting in and allow the fluid level to move.
If you really want to worry and go down a rabbit hole, buy some Castrol React SRF, it's £50-£60/litre and doesn't even meet the full DOT4 spec (but has a wet boiling point of 270C, higher than the Dry boiling point, 260C, in the DOT5.1 spec). I'm sometimes tempted but I don't think I know anyone else geeky enough to want to split it into smaller bottles! Whereas RBF-660 is half the price per liter and also comes in smaller bottles. And TBH when we're talking about getting the brake fluid to ~300C, mountain bike pad compounds simply won't be designed to get to the temperatures that could conduct that much heat through the piston (think about track car/bike rotors glowing red and sparks flying off the pads, just doesn't happen on a MTB). The more likely mechanism for vapor lock on MTB's is a poor bleed trapping some air in the caliper/hose that then expands enough to lock the brakes. Or people overfilling their brakes to try and reduce the free stroke meaning there is nowhere for the fluid to expand (it still expands as it warms up just like any material).
I keep my fluid stocks in ziplock bags anyway, so probably all fine then. I have managed to discolour rotors in the past and according to a steel temper colour chart they must have reached 250 ish degrees C. I doubt that temp transferred to the fluid though and in any case it was brand new fluid at that point, sealed in the system.
Most of the water will be getting in through the caliper seals as they actually move unlike car or motorbike brakes
MTB brakes work in exactly the same way as car or motorbike brakes. The pistons do not slide in and out of the seals every time you apply the brakes. The seals flex when you apply the brakes and then spring back to their original shape, which retracts the piston. The pressure of the brake fluid is far above any pressure of some water that might splash around when you're riding. Unless you point a jet wash right at the pistons, you aren't going to have water getting in through the caliper seals.
If you keep your fluid in an airtight container, it won't absorb moisture from the atmosphere. Flushing and bleeding the brake fluid when you change pads is really all you need to do.
MTB brakes work in exactly the same way as car or motorbike brakes. The pistons do not slide in and out of the seals every time you apply the brakes. The seals flex when you apply the brakes and then spring back to their original shape, which retracts the piston.
I know, but on a car or motorbike the pistons don't (barely) move at all, they just apply pressure. On a (push) bike they move out 0.5-0.8 mm or so. The seals flex far and deform more than on a car even if they don't actually slide.
And the other 99% of the time when the brake isn't pressurized they're covered in mud and water which will work it's way through the seal.
Bleed a motorbike after thousands of miles, or a car after tens of thousands and the fluid is still cleaner than MTB brakes that have probably done a few hundred miles. And all that crap is always in the first flush from the caliper, not the reservoir.
Bleed a motorbike after thousands of miles, or a car after tens of thousands and the fluid is still cleaner than MTB brakes that have probably done a few hundred miles.
A car can do tens of thousands of miles in a year. Most MTBs only do a few hundred miles a year. Car systems hold an order of magnitude more fluid than an MTB system. Replacing fluid once a year is normal for cars and MTBs.
A car can do tens of thousands of miles in a year. Most MTBs only do a few hundred miles a year. Car systems hold an order of magnitude more fluid than an MTB system. Replacing fluid once a year is normal for cars and MTBs.
Precisely my point, and the fluid in most MTB's I've ever bled after a few thousand miles at most is filthy, and I've never had a car with recommended annual changes. After the usual 2-3 years it's at worst got a slight golden-brown tinge in the first bit out the calipers.
If someone says "I've not had to touch my brakes in 2 years riding" we'd take that as an excellent review. On a car that's just normal before you do some preventative maintenance.
Car's run at upto 1400psi, SRAM seem to be the highest in MTB at ~850psi, and magura and formula are right down at 550psi.
You can wiggle MTB pistons out with your fingers, car's you won't shift without an air compressor.
They're very different use cases.
In a car I could well believe that testing the fluid in the reservoir (which as I said, unlike a MTB brake is possibly even open to the atmosphere, not bled of any air like the bladder in bike brakes) would give a representative view of how much moisture is in the system. Taking the reservoir cap off MTB brakes and testing the fluid in there would be pretty pointless.