A question from a member:

Originally Posted
would like your opinion on mixing DOT 4 and DOT 3 brake fluid.is this advisable? i plan on flushing the system as much as possible and not sure if i used DOT 4 last time but bought 3 this time.its been 3 or 4 years and i cant remember which i used...thanks.

Senor Honda answers......
If the DOT 3 is unopened, return it for a refund or get enough of DOT 3 to thoroughly flush the lines. If you think that DOT 4 is better and the DOT 3 cans are not opened, return them.
In either case thoroughly flush out the current fluid. Brake fluid is fairly cheap. If you get it from Walmart, it's even cheaper. If WM sells DOT 4, it must meet the federal DOT 4 standard whether it's the store brand or not

I never mix brake fluids. I thoroughly flush the lines. I follow the book as to which order the lines should be bled. When the master cylinder is almost dry, use a lint-free clean cloth and q-tips to thoroughly clean any dirt from the master cylinder bottom inside.

Clean the filter that is inside the master cylinder. I use brake cleaner from Walmart....lots cheaper than the name brands. I spray/clean the outside of the master cylinder especially around the plastic cap and the cylinder top.. Usually by the time I empty the second quart, there is only new fluid in the lines.

Reading the label is a pretty good guide.
Some MFG'ers may have compatibility within their own brand.
If the DOT 3 is old and you suspect it has been polluted by moisture, toss it out. Brakes are too important to take a chance.

I would get about two quarts of fluid plus about three small cans. Use the quart cans first. If you don't use all the small cans, take them back and get a refund if they are unopened. If they are opened, pour it all into the master cylinder and bleed it on out.
-Bob

 

The difference between dot 3 and dot 4 is the boiling temperature. Your car shouldn't get damaged if you mix them. If you're stranded somewhere and need some brake fluid and you can only find 3 and you use 4 or vice versa it wouldn't damage your system.

 

dot 3 and dot 4 fluids can be mixed.
Simply put, dot4 starts off as dot3, then borate esters are added.

Generally speaking, you can mix any dot 3 with any dot 4. However, there is some concern about different brands mixing together. Some of this might be due to marketing, some of it might actually be based in truth. Personally, I try to use the same stuff all of the time. When I change fluids, Typically there is a substantial color difference, and I continue to flush the system until what is coming out is the same as what is going in. I figure that vast majority gets changed then, and anything that is left is negligible.
Worst case, you're looking at lower boiling temps then you thought, and shorter fluid life. But since brake fluid should be changed regularly anyway, this isn't a huge issue.

dot 5 is the fluid that cannot be mixed as it is an entirely different chemical compound.

 

Dot 3 and 4 have different boiling points for wet and dry. Dot 5 iirc the chemicals in it cannot be used with vehicles that have ABS systems.

 

yeah, that's due to that added borate esters. different amounts make for different boiling points.
since the base is the same, it is safe to mix the two. You'll just, basically, average the two boiling points together.
DOT5 is silicone based. the problem with silicone is that it compresses slightly under pressure. Because of that, when the abs pump does its thing, it compresses some, allows for air space, then expands back, and repeat. After a few cycles of that, you've created foam, and as you can imagine, that isn't good for ABS systems.
Now,
DOT5.1 is a different story. DOT5.1 starts its life out as DOT4.
DOT5.1 basically has a higher boiling point, but lower life span since it absorbs water more quickly. There are a few other differences too. Generally speaking, unless the manufacturer recommends DOT5.1, there really isn't any reason to use it since there are many DOT4 fluids that have very high boiling points, wet and dry. DOT5.1 would really only be useful in super crazy brake systems on purpose built cars where you only need the fluid to last through a race or two, and then the whole system gets flushed.