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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.
Personally i would let all the fluid out the bleed screw before removing the mc to reduce the mess. I think youll lose all the old fluid in the install/bleeding process anyway. If your asking about the residual dot 3 left over i wouldnt worry much.
Edit: that being said i ran dot 3 in my slowly driven daily bc i had some. Like noted below its just the boiling point thats different. No issues
That's what I thought, but the cap made me doubt myself. The fluid in the system isn't even old. I just want a slimmer profile of the m/c to go with the rest of the build.
Right. Not to worry. DOT 3 and DOT 4 fluids should mix fine with no adverse side effects, they are the same sort of chemical base (polyethylene glycol) and the only functional difference is the boiling point as already noted. If you don't drive like a road racer on the street it likely makes no difference at all. Boiling brake fluid has a pretty much instant effect of making brakes stop working, but if you don't generate that much heat hot rodding your bike there is no difference in function whatever. There are no "DOT 4" parts in your system that need DOT 4 fluid - the manufacturer simply felt the need for the higher boiling point safety factor as it evidently is possibly to use the brakes to this level. In any event, simply take Hillsy's advice and purge all the DOT 3 fluid out in the bleeding process. It won't matter if a little DOT 3 fluid somehow remains, it is very nearly the same stuff as the DOT 4, and there is no chemical issue at all with a bit of a mix.
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