5.1 fluid
 

I'm just putting my rear brake back together and just wanted to know if using 5.1 fluid was ok to use? I have read a couple of articles about brake fluids and it would seem ok? Just wondered what you guys thought about it??

Cheers

No do not usr 5.1. Use Castrol SRF

Why? DOT 5 is the silicone based fluid that should not be used.

DOT 5.1 has twice the boiling point properties of DOT 4 and is fully compatible.
I've been using it on my bike since I started in DD in 2008.

Castrol SRF is great for race cars where the fluid can reach extraordinary temperatures, but on our little bikes to be honest DOT 4 is satisfactory too.

SRF is dot 4 as for dot 5.1 you'll be changing more often but if you've got the money then go fr it.

I know it is twice the price of DOT 4 but SRF isn't cheap either. I only as a matter of course change the fluid mid season, so twice a year, 250ml is hardly going to break the bank.

Use 5.1 and change once a year if it's lucky. Maybe it's because I don't use the brakes :burn:

You're right. I like SRF which isn't cheap either

DOT 5.1 Brake Fluid
 

The DOT specification for DOT 5.1 brake fluid has a higher minimum dry boiling point/minimum wet boiling point (527°F/347°F) than the DOT 4 spec (446°F/311°F), but these are just minimum specs.

Several DOT 4 fluids exceed DOT 5.1 specs. For example, expensive Castrol SRF (590°F/518°F) is used by Formula One teams, ATE Type 200 and Super Blue Racing (536°F/392°F), and Motul RBF 600 (593°F/421°F). Golden Spectro Supreme DOT 4 (520°F/367°F) comes close.

So DOT 5.1 fluid is not necessarily better, particularly after absorbing moisture. Most of us don't change our brake fluid very often so wet boiling point numbers are more important. On the track, brake fade is the main concern so we change the fluid more often and use a higher dry boiling point fluid. Generally, the higher the dry boiling point, the faster the fluid absorbs moisture from the air, Castrol SRF and the ATE fluids excepted.

If you don't change your brake fluid once a year or so, the absorbed moisture in the fluid corrodes expensive brake bits. So, do it this spring.

Castrol SRF is the best, hard to find, four-times more expensive than the others, and used by every Formula 1 team in existence because it’s boiling point drops very little as it absorbs moisture. (590°F/518°F)

Motul RBF 600 is next best and reasonably priced. Same dry boiling point and the next-highest wet boiling point. It tends to absorb water more quickly than the others so it seems to be best suited for racing applications where the fluid is changed very frequently. Keep in mind that any fresh dry fluid will be better than wet Motul because it will have a higher temperature boiling point. (593°F/421°F)

This suggests to me that ATE is the best all-round brake fluid.

BMW recommends ATE brake fluid for their street cars because it, like Castrol LMA (446°F/311°F), absorbs moisture very slowly. The advantage over LMA is that ATE has a much better wet boiling point. You can use this in your street bike and forget about it for a year or so. An excellent choice for a weekend track bike which also sees regular street duty.

ATE Super Blue and ATE Type 200 and are the same fluid. The theory is to alternate them and you can tell from the color when you have flushed all the old fluid out. In practice however, once a can is opened it should be used-up quickly. It’s not a good idea to save fluid from an opened can for color change reasons. (536°F/392°F)

As a point of reference, Motul 5.1 is 518°F/365°F.

Go you know if there are any compatibility issues in swapping from DOT 4 to 5 or visa versa?

DOT 4 fluid can be replaced by DOT 5.1 fluid and vice versa.

DOT 5 fluid is different. It is silicone-based and cannot be used in Ducatis that use Brembo brake and clutch master and slave cylinders. It will damage the natural rubber seals. It is dyed purple to give a visual warning against accidental use.