[Closed] Which dot fluid?

4 and 5.1 are compatible. TBH there's not really that much between the two, and if your brakes boil it's either because they're horribly wetted out, you're dragging all the time or the parts are crap, not because of your choice of fluid  so don't get too het up. A top end dot 4 can be comparable in performance to a low end 5.1 anyway.

For myself, I have good brakes so it doesn't really matter at all what's in them when it's fresh, but I use cheap 5.1 because it has a longer service and shelf life- used to use Halfords own as it was a nice cheap 250ml bottle but it's discontinued so now I buy the same stuff branded as TRW, or Hope stuff when I see it as it's all the same.

As said, 4 or 5.1, whichever is cheaper at the little motoring shop round the corner from me. Lol

I favour motul rbf600 for its wet boiling point. It's a DOT 4 but better spec than a lot of 5.1 stuff. Not the cheapest and the smallest bottle is 500ml but I have several bikes to do. When a new bottle gets opened because one needs a service or something, then all the brakes get flushed.

As people have said - DOT 4 or DOT 5.1 it won't make much if any difference.

Biggest change for DOT 5.1 is a reduction of the low temperature viscosity. This improves ABS/DSC/ESC response speed on car systems.

I thought the viscosity thing was a separate standard, I.e. you could get low viscosity DOT4 for ABS?

As Onzadog said, the 4 and 5.1 specs are a minimum requirement mostly based on dry and wet boiling points. In reality unless you have serious issues your brakes will never get near the amount of water the wet boiling point is measured as because MTB brakes are sealed whereas car brakes are open to the atmosphere and we're more likely to bleed our brakes yearly rather than every few years in a car).  So a good DOT4 will have a higher dry boiling point than a normal dot5.1 even though the minimum dry boiling point for 5.1 spec is higher. This is because the formulation required to prevent boiling when wet has the trade off of a lower dry boiling point.

All that is mostly moot though as our brakes with plastic and ceramic pistons don't tend to boil fluid anymore like older brakes did. You'll get fade from the pads long before you boil the fluid. Although boiling fluid tends to be a cumulative effect on longer descents whereas fade happens as soon as the brake pads get too hot so you can boil the brakes with lots of dragging.

Not 5.0 !  This is crucial as it’s not compatible and would be as bad as using mineral oil in a DOT system.

I reckon for practical purposes most won’t see any difference between 4 and 5.1 in a blind test, but as there shouldn’t really be any price difference I’d get 5.1 if I had the choice. Just because.  I’ve got a bottle of 4 on the go at the moment and it really doesn’t bother me.

i thought dot 4 was able to take more moisture than dot5.1 ?

Thanks all... I'm heading out to buy a 5 gallon drum then, should be enough 👍

I may have bought some dot4 at wilco last week something like 500ml for  £3.99.

Its probabably marked on the shifter when you pull back the rubber but you can mix 4 & 5.1

Probably not the greatest stuff to keep around the house anyway its evil stuff and if bleeding you wont use much.

Don't forget its a paint killer btw.

A 500ml bottle will last most people over a year in mtb use.   While it’s in the bottle it’s fine, just clean up spills and wear gloves.  IRL, years of exposure to brake fluid while a teenager with crap cars before everything was dangerous hasn’t killed me etc so while it’s definitely not good for you, it’s not like it’s Novichok or something.  Paint damaging powers are a thing though - wash any spills as soon as you are finished.

It does absorb atmospheric water while sat in an open bottle. Ideally you shouldn’t use opened bottles of DOT fluid but the amount used for a bike (maybe 60ml) it’s inevitable.  Probably worth putting foil over the neck before putting top on or keeping in a ziplock bag etc to try to improve the situation.

Dirty oil including brake fluid should not go down the drain. There’s a tub for it at your local recycling centre.  I have a marked container for dirty brake fluid that I empty when appropriate...

 Probably worth putting foil over the neck before putting top on or keeping in a ziplock bag etc to try to improve the situation.

In a plastic bag with silica gel sachets in it (the bag) I have been told.

Just to make point; the 'wet' boiling point is with 3.7% water, i.e. just over an espresso shot of water in a liter bottle. Even with just a loose screw on lid it will not absorb that in years sat on the shelf unless you live in the tropics and leave it outside.

Remember it lasts perfectly fine in your car for years which probably has a vented cap on the reservoir and lives in the warm/wet engine bay.

Good quality DOT4 is far preferable. Motul RBF or ATE SuperBlue are well respected.

Remember it lasts perfectly fine in your car for years which probably has a vented cap on the reservoir and lives in the warm/wet engine bay.

Most cars actually have minging black brake fluid way past its best because once its a few years old nobody ever does the 2 year fluid change reccomended by most manufacturers!

Buy a bottle of ATE Dot 4, do your brakes, then do your car too 😀