MikeDee wrote:
DOT fluid has been used in automotive brakes since the 1950s I think (longer than I can remember); long before ABS brakes came out.
DOT fluid (by which I mean DOT3,4, 5.1, not DOT 5.0) was first used in aircraft systems. It was never meant to be a fit and forget option there and its low viscosity was a benefit in the long (narrow bore for weight reasons) pipe runs, just as it remains a benefit in modern ABS units in cars. [BTW DOT 5.0 is a silicon oil and it isn't as good in ABS units. It does last a lot longer in the system before it causes corrosion and fails though.] DOT fluid was used in car disc brakes because those systems were first (literally) taken from aircraft and bolted onto cars. Early disc calipers for cars often used the same caliper bodies as were fitted to aircraft with fixed undercarriage, so that they were aerodynamically shaped, even.
I've had automotive brake and hydraulic systems apart and saw zero corrosion in a clutch master cylinder that used DOT fluid that was made of aluminum after 10 years of use.
One swallow doth not a summer make. I have probably spent more time fixing duff hydraulic systems on cars than any one other thing. Quite a lot depends on how the system is sealed and what temperatures it sees in normal service. DOT fluid is so hygroscopic that an open container will absorb enough moisture from the air
in one day that it will be out of spec, (with a severely lowered boiling point). It is highly educational to get a moisture tester for the fluid and to use it; the fluid can go out of spec though moisture absorption in a few weeks. I've seen many systems (in which the MC reservoir wasn't well sealed enough or didn't regularly get hot) in which the system corroded
internally after a relatively short period of time. Every maintenance schedule for cars from leading manufacturers demands that DOT fluid is changed completely at intervals that are around a year or eighteen months. Moisture permeates through flexible hoses and past seals to contaminate slave cylinders. The usual thing is that the fluid in the slave goes black and stinky inside a year and after that corrosion may cause failure at any time. Most folk get away with it if they don't bother but the frequency of these failures (in cars that are say 3-5 years old and are not maintained correctly) is high enough that if they occurred during the warranty period then the cars would be recalled. Dual circuit brakes are the only thing that makes this sort of technology safe enough to use at all, in fact.
If you get fluid on a painted surface, just flush it off with water. I don't see this as a biggie to not use the superior fluid.
If you get a leak on a bicycle system the first thing you know about it is that the paint has gone from the parts nearby the slightest leak. It is a very effective paint stripper. I'd be interested to know why you think DOT fluid might be 'superior' because as far as I can tell it isn't, not for bicycles.
Perhaps you misunderstand where the corrosion that causes the caliper failures in this thread comes from; in the usual case a small amount of (probably salty) water gets in the caliper bore and then corrodes the caliper piston seal seat. The slightest weep of mineral oil that at first results cannot stop the corrosion but it must slow it down a little. In DOT systems the leak rate is instantly x10 faster (because the fluid is less viscous) and worse yet it
accelerates corrosion because it is perfectly miscible with water. [For example in many cars a MC leak of DOT fluid betrays its presence by stripping the galvanising off the outside of the vacuum servo, it is that corrosive.]
I haven't had my Hayes MTB brakes apart but will report back on corrosion issues if I disassemble them. They use DOT fluid.
In several LBSs near me Hayes brakes are not referred to as 'Hayes brakes' but as 'those S**t Hayes brakes'. In truth they are no worse than a lot of other DOT brakes (most of which work OK as MTB brakes that see relatively little use in mostly non-corrosive conditions; I have DOT brakes on an MTB too BTW) but in the UK Hayes brakes tend appear as OEM on mid-price bikes that may also see a fair mount of winter use on the road, with exposure to road salt. They fail very often as a result; it definitely isn't worth trying to fix them at workshop rates so they get binned. Consequently pretty much no-one buys Hayes brakes aftermarket in the UK; their reputation (like the brakes often are themselves) is badly tarnished..... The spare parts are not easy to get hold of, expensive for what they are, and the wholesalers tend to sell off any old stock in one go which effectively dissuades any LBS from stocking parts at full price; their stock may be become worthless overnight.
Fixing bicycle hydraulic brakes (for mid-price MTBs) is hardly ever cost-effective in a workshop; you can buy a complete pre-bled brake for cheap (that comes with a two-year warranty) and fit it in fifteen minutes. Resolving any hydraulic problem (other than that can be dealt with by bleeding) takes a lot longer and it comes with uncertain results. Not a good combination in a workshop; you soon exceed the value of the brake you are working on and you stand a good chance of just getting a herd of annoyed customers.
For me the bottom line is that If you insist on using hydro brakes on the road in a UK winter, you can expect them to fail, and fail quite often.
Definitely more often if they are filled with DOT fluid than mineral oil. It is probably worth spending five minutes smearing silicon grease on extended pistons in new brakes, because that might keep the corrosion at bay for a bit longer. On used brakes you would have to be sure that you were not just trapping moisture etc behind the silicon grease.
cheers
Not that I'm recommending any such thing.