Would appreciate some wisdom from the STW massif.

Two questions:

1. Which is better and why, dot 5.1 or 4 (hope mono minis)?
2. Can you tell which fluid is already in the cylinder eg by smell or appearance or do I just have to flush it and start again?

Cheers peeps.

The 5.1 supposed to have a better heat tollerance. You can readily mix 4 and 5.1 – I put 5.1 in as I can get loads of it cheap in Halfrauds. There is a difference in density when you mix the two together like a salt solition and water etc but I dont think you can get an idea of what it is from the appearance.

Nice one thanks for that

Just don’t go using any DOT 5

What happens then?

DOT 5 is silicon based and totally different from DOT 4 and DOT 5.1. Bit like mixing mineral oil in a DOT system.

I always use 5.1 in my Hopes and Avids. A 250ml bottle from Halfrauds is pretty cheap. It doesn’t store indeffinately so you might as well flush it through so it’s all 5.1 and then pour the waste back in the bottle and take it to be disposed of properly.

The reason it doesn’t store i believe is that it absorbs air and thus will be more spongy. I may be wrong but i believe 5.1 is more heat tolerant but is also absorbs the air quicker and hence requires changing more…

it absorbs water [from air]and therefore its boil point reduces

Thank you junkyard you are very much right it is moisture not air! Old age getting to me!

I’d not flush it all back into the bottle myself (seeing as it comes in litre bottles and you use about 50ml) – I’ve had no problems working with 5.1 that is 5 or 6 years old from a sealed container, the problem with a car brake system is that it is open at the master cyl end and therefore has an infinite supply of moist air – hence it absorbs lots of moisture in the car. If you just open the bottle, pour some out and re-seal it well you will find it still works fine for years as the amount of moisture introduced into the bottle is minimal. The only time it becomes more spongey is if you manage to boil it by overheating your discs. Not many people see those sorts of temps on bike brakes in this country, and those that do should be bleeding brakes more regularly anyway as a matter of course IMO.