I'm going to try and flush the brake fluid in my truck. I'm sure it's never been replaced. Is it ok to use Synthetic as I flush? Is it a problem if I don't every drop of old fluid out, is it ok that the two mix together?

Also, while I'm at it, I will be doing a trans flush soon.

Same thing.. is ok to mix?

 

yes... you can mix them as long as they are both DOT 3 or whatever your vehicle calls for. Same for the tranny.

 

Unless they have come out with a Dot 3 Synthetic don't do it.
The only synthetic I've seen is Dot 5.

Most tranny fluids are compatable.

 

So dot 3 and dot 5 are not compatible?

 

^ correct, DOT 3 and DOT 5 are not compatible.

 

Hence... you can mix'em if they are both the same DOT rating.

 

I double checked, I got DOT 3 synthetic.

Sucked out the fluid from the master cylinder resevoir... was almost black as oil.