Is that ok, or will he need to clean it out and start over? What will it do to the system if he leaves it mixed?
yes you can mix them it should be fine if he is worried you can bleed the system out with fresh fluid You can however NOT mix DOT 5 with dot 3 or 4
i think dot4 system use dot4 because it works better under higher temp conditions. depending on what car it is or how it's driven it could be fine. i would flush the whole brake system
Am I right in thinking that dot 5.1 is compatable with dot 3 & 4, but as you say dot 5 is silicon based?
When he first told me he did it, i was thinking dot4 was the silicone and he added the glycol-based 3 to it, and I just about flipped. But I pulled the internet up on my cell phone and found that 5 is the silicone and told him i "think" it is ok, but would ask around to make sure. thanks for all the feedback.
the only concern would be if he is using the brakes alot in performance conditions. if hes doing that or something that generates alot of heat then just use a turkey baster or something similar to suck the fluid out of the mastercylinder then refill with dot 4. if hes not doing anything but normal driveing then he will be fine.
Used to be DOT4 was used in British cars and if you used DOT3 in those cars it would eat all the soft parts. As far as compatability, sounds like he's good from what everyone else posted.
You are confusing DOT 4 with castrol based brake fluid. Those two are NOT compatible. The dot 2,3,and 4 are all completely compatible - though better if forward based (going from 3 to 4). Your dad is fine as long as (has been said) he is not in competition with it or doing a lot of high speed braking that generates a lot of heat.
