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I would describe a braking system as closed but not a closed loop. If your braking system is open you've got a problem because your fluid is free to leave. That would be bad. Many braking systems don't return to atmospheric when the peadl is not depressed. The MGB for instance maintains around 8psi in a mixed disc/drum system.
When you have the brakes applied, which is when you produce the heat, the fluid is under anything up to around 2000psi. And 120 sounds way low for brake fluid to boil, hence my question.
120 sounds way low for brake fluid to boil
Magura fluid (and Shimano) is mineral oil, which has a much lower boiling point than proper DOT brake fluid. For reasons that I don't understand, people think mineral oil has benefits over proper brake fluid. Overall, I'd much rather have a system with DOT fluid.
DOT brake fluid is mildly nasty stuff if you spill it, it will dissolve paint and is toxic, and it absorbs water if exposed to the atmosphere. When it does absorb water, the water will be distributed throughout the fluid and will reduce the boiling point. However, even DOT fluid with quite a lot of water in it still has a higher boiling point than mineral oil. On top of that, if water gets into a mineral oil system, it will gravitate to the lowest point, which is generally the caliper. The most likely mechanism for it entering the system is working its way through the caliper seals as they flex when you apply the brakes in heavy rain, during water crossings, etc. If this does happen, then the droplet of water in the caliper will boil at 100 degrees Celcius, so it will flash to steam if you are applying the brakes hard and then release them. Mineral oil isn't as harmful to paint as DOT fluid, but it is still toxic, so I don't really see much advantage on that front. On top of that, you can buy DOT fluid at any petrol station, whereas manufacturers of mineral oil systems insist that you must use their special oil (which isn't really special, it's just a hydraulic fluid with a different label on the bottle).
Brake systems are open in the sense that there is a reservoir that is at atmospheric pressure. There is a rubber bladder above this to allow you to turn the bike upside down, but the reservoir itself is not pressurized. When you apply the brake lever, the port to the reservoir is closed and then pressure builds up in the brake system. When you release the lever, the piston in the master cylinder retracts past the port to the reservoir and fluid can transfer between the master cylinder and reservoir. As soon as this port is open, pressure in the system drops to atmospheric. This means that the boiling point is lower once you release the brakes, so water in the system can flash to steam and drive fluid out of the system and back into the reservoir, resulting in brakes that don't work.
Drum brakes require a small amount of residual pressure in the system to prevent the wheel cylinders from leaking. This is not relevant to disk brakes, the seals work in a completely different way. Even then, the small amount of pressure is not enough to make any substantive difference to the boiling point of the fluid. Fluid must be able to return from the wheel cylinder to the master cylinder when you release the brakes or else the brakes would bind, so this residual pressure is massively lower than operating pressure. Systems like this are still open.
Did you work any of that out or just copy and paste it from here to look scientific?
Amazing 😂
Shimano Mineral oil has a higher boiling point than all DOT fluids even before they’ve absorbed any water.
https://www.epicbleedsolutions.com/blog/dot-brake-fluid-vs-mineral-oil/
Sorry, when I said "mineral oil" before, I was referring to the Magura stuff, which has a strangely low boiling point.
One thing about DOT fluid is that the specifications are the minimum requirement, so DOT 5.1 is required to have a dry boiling point of at least 260 degrees Celcius. Good quality DOT fluid will exceed this. As a random example, Castrol SRF is sold as a DOT 4 fluid, but advertised as having a wet boiling point of 270 degrees Celcius and "the ability to withstand temperatures over 300°C". I don't know where the claimed boiling point of Shimano fluid comes from, but the real world benefits are pretty much limited to not melting the paint if you are careless and don't wash it off after you bleed your brakes.
For the record, my bikes have a mix of Hayes El Camino, Hayes HFX, Shimano Deore, and SLX brakes. They all work fine when they are properly bled.
Sorry, when I said “mineral oil” before, I was referring to the Magura stuff, which has a strangely low boiling point.
Hang on. According to the STW "experts" all mineral oil is the same.
I've made a complete mess with brake fluid many times and yes, it only damages paint if you don't wash it off. At least, the paints on which I've spilled it. Also ruins car tyres if you don't wash it off.
I reckon the bladder in most brakes is good enough to seal the system so no water vapour gets in ever. I go many years without bleeding Hope brakes, they still work well.
Depends on if it's a fiber or sintered pad.
Fiber based pad. No. You'll die.
Sintered. Yes, and you'll get more power.
Works of course only with metallic or sintered pads. Big no-no for organics, Kevlar and likes.
Why? They're brake pads, they are designed to get hot. Cooking them for long periods probably isn't a good idea but sticking them in a flame for long enough to get them smoking won't do any harm.