What's the best way to do this (Avid and Hope). Do I need to somehow remove all the 4.1 first, or can I just flush it through with 5.1, or will this cause the 2 oils to mix? Any ideas roughly how many reservoirs of oil will be needed to flush the rear brake?

cheers

Dot 4.1 ?

DOT 4 and DOT 5.1 are similar.
DOT 5.1 is really for motor vehicles that have ABS, but is OK for bikes.
There is so little fluid in bike brakes that if you bleed them at all it will be enough to flush them.

Just bleed the 5.1 through. There'll be a little bit of mixing where the 5.1 and 4 meeting the pipes, but not so much that you'd worry about it. DOT 5.1 and 4 are compatible. Never heard of 4.1 though 🙂

It's pretty rare for someone to completely drain a brake system – only if going from glycol to silicone fluid (ie, DOT4 to DOT5) and changing all the seals. Even cars are done by putting the new fluid in whilst draining the old fluid out.

im no expert but after watching the dude on the Hope website (how to videos)…id stick with just 5.1 fluid…. get rid of the 4.1..just to be sure !

Meant Dot 4, rather than 4.1

cheers

Just to add to the discussion (sorry), I'd keep the DOT 4. It's less hygroscopic (absorbs water less easily), so lasts longer and needs bleeding less often.

DOT 5.1 is really for high performance applications. If you've never over-heated your brakes on a long Alpine-type descent there's no need for it.

APF

Yeah, for most users 4 is sufficient but actually more expensive in my local motor factors (though I can't say I've ever needed to bleed my brakes other than when I damaged a line, in years, other than when I boiled a 10 year old rear mini on an alpine descent and that had 4 in it from birth.

?????

Theres some boloks spouted up there, either that or everything I know about brake fluids is wrong.

The difficult bit of the standard to meet is the wet boiling point, this requires the addition of more additives, which in turn lower the dry boiling point.

Some track day cars and single seet racers therefore use dot3 as it has a higher boiling point than dot 5.1 when dry, but is ruined fairly quickly by water so would be useless on the road. As dot3 is as rare as hens teath in this country your best bet is a good quality dot4 (look for a brand with 2 different containers, one probably says dot 4, the other dot4 performance/raceing/competition) and replace it annualy. dont worry about leaving it in the container for long periods, unless your leaving the lid off theres no water in the container for it to absorb anyway so your probably good for a fair few years.

web linky : http://www.shotimes.com/SHO3brakefluid.html

Doesn't discuss dot5.1 , but you get the general idea.

tinas – not sure what bits you're referring to that you disagree with. I've never yet been presented with any info on the hygroscopy rates of DOT fluids (I'd love to see them) but

Some track day cars and single seet racers therefore use dot3 as it has a higher boiling point than dot 5.1 when dry,

I think your thoughts on the matter are wrong…
DOT 3 205 °C (401 °F) 140 °C (284 °F)
DOT 4 230 °C (446 °F) 155 °C (311 °F)
DOT 5 260 °C (500 °F) 180 °C (356 °F)
DOT 5.1 270 °C (518 °F) 190 °C (374 °F)

5.1 is better in all extremes, other than possibly hygroscopy but since I've never seen any info regarding it it's hard to comment. The DOTs are boiling performance standards, 5.1 being the highest so far. DOT5.1 on wiki will link you to the official DOT standards on the US gov website which shows you 3, 4 and 5 (but not 5.1) but they're all increasing in temp on both wet and dry, and I've never yet seen 5.1 rated with lower boiling points than 3.

coffeeking – Member

………. in years, other than when I boiled a 10 year old rear mini on an alpine descent and that had 4 in it from birth.

Thats why it boiled – old fluid.

Dot fluid should be replaced regularly – every year preferably. it costs pennies and protects the system.

The standards are a minimum (so probably what you'll get from a cheep bottle of dot5.1)

A good bottle of dot 3 or 4 will be a higher dry boiling point than a good bottle of dot 5.1.

edit: so yes, in your case coffeeking if you bleed your brakes every 10 years 5.1 might be better, but for people who bleed them, at a minimum annualy, a high quality dot4 will be a much better fluid. Or if you can get it a good dot 3.

You can buy high performance / race DOT 4 fluid with a higher boiling point than normal DOT5.1.

But compare the stuff that's vaguely normal and you'll find that things are exactly as Coffeeking says – as the numbers get higher, so do the boiling points.

im a Hope user and mine is definately 5.1 fluid !

The standards are a minimum (so probably what you'll get from a cheep bottle of dot5.1)

A good bottle of dot 3 or 4 will be a higher dry boiling point than a good bottle of dot 5.1.

edit: so yes, in your case coffeeking if you bleed your brakes every 10 years 5.1 might be better, but for people who bleed them, at a minimum annualy, a high quality dot4 will be a much better fluid. Or if you can get it a good dot 3.

With respect, I think that's a little like claiming semi skimmed milk is full-fat. If it meets the criteria for full fat it IS full fat? They did sell "super dot 4" but doesn't quite reach the dot 5.1 requirements.

Wynns, makes of brake fluids, say:

The United States Department of Transportation (DOT) sets
standards for brake fluid performance, largely based on the boiling
point of the fluid. DOT 3 and DOT 4 brake fluids are both polyglycol
based brake fluids with DOT 4 having a higher boiling point than
DOT 3 fluid and thus an extra margin of safety in operation. As
water content increases, DOT 4 fluids are also better able to
maintain their higher boiling point than DOT 3 fluids.

I think you're working from old wives tales, our single seat race car runs 5.1 and so do all of the other teams that enter.

Dot 3 replaced dot2, when they made materials that could cope dot 4 came out and replaced dot 3, then they made dot 5 which was marginally better boiling-point-wise but silicone based, and then they managed to blend a glycol based one that was even better than any of the others but 5 was already taken, so to aid confusion they called it 5.1.

Further reading suggests that all of the DOTs can be made to beat the dot5.1 DRY boiling point, but only dot5.1 meets the 5.1 wet boiling point. 5.1 is equally hygroscopic as 3, so no gains or losses there, but it costs more and if you replace it regularly it would be expensive to do. That said, I get it for £4 a litre, so it seems like a no-brainer to me, why bother with something with lower wet boiling point if something higher is available at half the price of a pint?

Theres some boloks spouted up there, either that or everything I know about brake fluids is wrong.

Well guess what?

Not from old wives tales, I did this at uni.

Dot 3/4/5.1 are essentialy all the same basic chemical (propyl glycol IIRC) which has a very high boiling point, but looses this as it absorbs water which is where silicon and oil based fluids are usefull, but they have their own issues.

If you add additives to the glycol is mitigates the effects of water content and gives it a more consistent boiling point. The downside of this is the dry boiling point is reduced.

I've no dobt that the dot3 available when the standard was introduced might have had a boiling point comparable to the standard. Ditto dot4 and 5.1 but whatever development castrol/mobil/whoever put into making really good dot5.1, the same developments push up the boiling point of dot 4 fluids as well as they have less additives.

Otherwise no one would sell or use dot4 (because brake fluid is cheep and if dot 4 really wasnt as good everyone would pay the £1 more for a bottle of dot 5.1.

Blimey! Almost sorry for my OP! I need to bleed 2 sets of brakes (gone spongey and 3 weeks in the Alps coming up) so thought I may as well go for 5.1 (as per Hope site etc). Seems that using 5 to flush 4.1 wont cause armageddon or a BP'esque environmental tragedy, so I'll go ahead.

Further reading suggests that all of the DOTs can be made to beat the dot5.1 DRY boiling point, but only dot5.1 meets the 5.1 wet boiling point

You're looking too much into wet boiling points and not taking into account the other things that make up DOT specs like corrosion resistance, viscosity etc! A lot of DOT 4 fluids have a much higher wet boiling point than DOT 5.1 😉

Cars with abs systems require a lower viscosity fluid which is the reason 5.1 spec was introduced.

Seems that using 5 to flush 4.1 wont cause armageddon or a BP'esque environmental tragedy, so I'll go ahead.

NOOOOOOOOOOO
5 is silicon based, you want 5.1 (glycol based)
and 4.1 doesnt exist (AFAIK)

A lot of DOT 4 fluids have a much higher wet boiling point than DOT 5.1

Not entirely true.
The last figures I can remember looking at, 5.1 has higher boiling points, but it can't absorb as much water as 4, meaning it becomes saturated at a lower moisture content, leading to the wet boiling point dropping of far quicker than 4.

5 was hailed as being the all new all signing all dancing brake fluid when it came out, but the boffins never allowed for the moisture issue in your typical vehicle braking system (all braking systems will suffer from moisture ingress, as it's near impossible to make and maintain a perfectly sealed system)
With a hygroscopic fluid (aka 3, 4 & 5.1), any moisture in a brake system will be absorbed pretty uniformly by the brake fluid, leading to a gradual drop in boiling point.
With a non-hygroscopic fluid (aka 5), any moisture will gather in the braking system, and gradually settle to the lowest point (usually the brake caliper/cylinders). This then leads to a drop of water with a boiling point of 100degC within the hottest part of the brake system.

quite pathetically, i’ve only today bled my brakes for the first time. 😀

less pathetically, it went very smoothly and successfully. two questions, firstly – what am i supposed to do with the old fluid that ends up in the syringes? and secondly, i take it you can’t reuse it?! 😆

old fluid – local tip will have an oil recycling facility – get rid there

Crikey – not going to read all that but dont bother worrying about it – just do a normal bleed with 5.1 and just ride the bike 🙂

That’s a YEAR ago! Damn – best read the post then 😉 I find burning it on the BBQ to get the BBQ going works well 😉

yep i did a search for how to get rid of old brake fluid, this thread came up so figured might as well ask here!

You don’t need to re-bleed every year, that’s rubbish.

Got some factory bled Juicy 3s which keep on working – and they’re nearly 3yrs old.