Brake Fluid

[bstig60]

Will it hurt to mix synthetic Dot 4 with regular brake fluid in the GL1800 brake system?

[baumedagn]

I don't know if it will hurt or not, but I would just flush out the old. Why take the chance?

[Viking]

All Dot 4 brake fluid is synthetic, as are Dot 3 and Dot 5.1. The only petroleum based fluid is Dot 5. They say you can mix any of the synthetics together, such as 3 with 4 with 5.1, however, you will get an unpredictable boiling point. I would NOT do it tho. But 4 with 4 is just fine. It has also been said that you can clean out Dot 3 and replace with Dot 4, but you cannot clean out Dot 4 and replace it with Dot 3. (?????? why not if both are synthetic and can be mixed together) There is a lot of good and bad info on brake fluid to be found, but the mixing of Dot 4 with Dot 4 is not one of the bad ones.

[tamathumper]

I just replaced all of my brake fluid with a big ol' bottle of "DOT 3+4".

So either I'm screwed, or it's from the future and it's actually DOT 7.

[Viking]

I just replaced all of my brake fluid with a big ol' bottle of "DOT 3+4".

So either I'm screwed, or it's from the future and it's actually DOT 7.

It is from the future and can be used in place of either 3 or 4. It is likely Dot 4.

[keithg64]

Actually dot 3 & 4 & 5.1are all glycol based and the 3 & 4 I know are interchangeable. The dot 5 is synthetic and is not interchangeable with the others. The 5.1 is a glycol base with a higher heat range.

[keithg64]

Here is some info on that.

https://auto.howstuffworks.com/auto-par ... fluid2.htm

[Viking]

Actually dot 3 & 4 & 5.1are all glycol based and the 3 & 4 I know are interchangeable. The dot 5 is synthetic and is not interchangeable with the others. The 5.1 is a glycol base with a higher heat range.

Actually "glycol based" does not mean they are not synthetic. It means they are synthesized from a glycol. Dot 5, in your link says Silicone based which is why it is not interchangeable with the other three. In my post above, where I stated that only Dot 5 is petroleum based, that also should have said SILICON based. Slip of the fingers before engaging brain.