Topic: Dot 3 and Dot 4 question

[Scott S]

 I have a functioning brake system with a master cylinder that says "Dot 3 or Dot4". The entire system has Dot 3 in it, from the m/c to the caliper.

 I want to swap out the master for one with a lower profile and it says "Dot 4 only". Do I have to flush the Dot 3 out of the lines? Can I just fill the new m/c with Dot 4? Dot 3 and 4 are supposed to be compatible, but the warning on the new m/c has me guessing.

[Flyin900]

The only difference is a slightly higher temp rating between 3 and 4. There is no chemical reaction possible from DOT 3 that will affect that DOT 4 master cylinder internals, so choose either sticking with DOT 3, or change to DOT 4.
Your system will need a full bleed anyway so put the fresh stuff through the entire system (your choice of 3 or 4) and don't worry about any residual DOT 3 in the lines if you switch to DOT 4.

[Scott S]

 And the Dot3 wont harm any seals or whatever in the new m/c?

[wowbagger]

Dino did a good explanation of the Dot 3/ Dot 4 fluid differences:

https://www.youtube.com/watch?feature=player_embedded&v=2yS8uw5-Rz8

Essentially, they are interchangeable. One just has a higher boiling point.

[Scott S]

 That's what I thought, but the warning on the cap made me doubt myself.

[alacrity]

I don't know exactly why Grimeca put that warning but my GUESS is engineering + legal dept.

Their in house engineers spec'd dot4 for their system because it generated a lotta heat in test conditions and they knew Dot4 would work and not fade.  So they didn't want customers adding Dot3 if the level went low, as the aggregate "Dot3.7" or whatever wouldn't have the same heat resistance. They wouldn't want a customer crashing because of brake fade and blaming (suing) them because they didn't specific Dot4 only.  Just a guess.  ;-)