A general brakes question, not to do with my own car (despite a brake thread a few days ago. ;-)
The hydraulic disc brakes on my mountain bike can use either Dot 4 or Dot 5.1 fluid. At the moment they're actually running on a mixture of both as I bled them a few weeks ago.
Apparently, Dot 5.1 is non-hydroscopic (if that's the word). So, if cars used this, would that eliminate the need for 2-yearly brake fluid changes?
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You're thinking of DOT5 not DOT 5.1 they are completely different and not compatible.
DOT 5.1 is similar to 4 but with a higher boiling point, the two can be mixed and both are straw coloured. DOT5 is silicone based and usually blue or purple. It cannot be mixed, it must be put in a 'dry' system. It does absorb water, but in some ways this is a bad thing as it can collect in pockets. DOT5 is usually used in vehicles which spend a lot of time not in use and indoors,
eg
Airport fire tenders
Harley Davidsons
Military equipment
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CORRECTION, I meant DOT 5 does NOT absorb water! but the 3, 4 and 5.1 do.
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this may help,
www.synlube.com/brake.htm
--
Steve
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Thanks. :)
Knew there must have been an obvious reason!
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Because Dot 5 does not absorb water,it will stay in the system as globules which are even more likely to become steam when they get hot.
How do they get into the system?
Master cylinders are ventilated-they have to be-otherwise the brakes would not work-there will be a certain amount of codensation due to changes of temperature and humidity in the atmosphere and engine compartment.Because the water is not miscible with the fluid and is denser,it will make it's way to the bottom of the master cylider and from there,as pads/shoes wear or are bled, into the pipework and eventually into the brakes themselves but as globules.
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So if master cylinders had a simple cap incorporating silica gel or similar water-absorbing substance, brake systems would never get any water in them?
It wouldn't be rocket science - even my anorak has little bags of silica gel in the pockets, I have just discovered.
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So if master cylinders had a simple cap incorporating silica gel or similar water-absorbing substance, brake systems would never get any water in them? It wouldn't be rocket science - even my anorak has little bags of silica gel in the pockets, I have just discovered.
A good idea on the face of it,but silica gel does not absorb moisture for ever,it has to be dried out when it has reached capacity.When I worked for G.P.O.telephones(yes,it was a while ago)if we worked in 'cabinets'(green junction boxes at the roadside)we would change te s.g.container if it had changed colour to indicate it was 'full'.
'Apparently, Dot 5.1 is non-hydroscopic (if that's the word).'
Almost-it is hygroscopic.
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I cannot buy into this water theory as too many classic cars use DOT 5.0 without any issues. Water is mainly absorbed via the brake hoses and DOT 5.0 would prevent this happening.
However, it is comprised of large molecules allowing air to reside in the spaces, which cannot be bled out of the system.
DOT 5.0 aerates with ABS systems, and gives a soft pedal so not an ideal brake fluid except for specialised niche markets.
DOT 5.0 Silicone fluid should not be confused with the DOT 4+ fluid Castrol SRF which is a silicon ester which absorbs water and used in motorsport.
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OK, here are some more suggestions:
1) As there is no net ingestion of air into the system, only respiration through the filler cap as the brakes are applied, then released, why not have a rubber diaphram (like a balloon)over the cap to isolate it and stop damp air coming into contact with the fluid?
2) If water really does seep in through the flexible pipes (hard to believe) why not have a very slight permanent positive pressure, not enough to operate the brakes,but enough to stop fluids moving the wrong way.
3) Why do they use hygroscopic alcohol-based products, and not oil as in other hydraulic systems - tractors, JCBs, jacks, etc?
4) What about a recirculating system, rather than push/pull? If the fluid circulated round the system with each pedal action, and there were return lines, it could incorporate a filter and water trap.
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I *think* Rolls Royce and Citroen use/used normal mineral oil for the brakss? I agree, it would perhaps be better, maybe ordinarily it is too viscous?
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The properties considered for brake fluid are
1. Dry boiling point
2. Wet boiling point
3. Kinematic viscosity (how thick the fluid is, with lower generally considered better for flow)
4. pH value (measure of acidity, with higher generally considered better for corrosion resistance)
5. Chemical stability
6. Corrosion
7. Fluidity and appearance at low temperature
8. Evaporation
9. Water tolerance
10. Compatibility
11. Resistance to oxidation
12. Effects on rubber
13. Stroking properties (lubrication capability)
14. Fluid color
Re the link, one aspect of pad fade is that at high temps the organic material in a pad produces gas and the pad will eventually ride on this film of gas, hence holes and grooves to delay this effect.
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