Need a quick answer. Thanks.

No

Dave

they won't mix. they're like vinegar and oil, they stay in seperate suspensions.

Can you add some DOT3 to DOT5 (or vice versa) to temporarily top off the MC?

Y E S ! ! ! !

Will they freely comingle their molecules? NO. They'll always separate out in to two fluids. There is no benefit from mixing fluids of different DOT ratings, but the Federal regulations that govern brake fluids REQUIRE that they can be added together and not affect the basic function of the brake system.

If you need to top off the MC to get the car home, do it. Then replace all the fluid with what you need ASAP.

Clair

YES they can mix DOT 3 and DOT 5 without any problems. if you mix the 2 type of fluids it will not be much better than the poorest quality fluid.
there was a post on this subject a while ago that had the federal law stating they had to be mixable.
when i change over to DOT 5 i just remove as much DOT 3 out of the master with a suction device. then i pour in the DOT 5 and bleed until i see the DOT 5 at the wheel cylinders. after i do a 4 wheels i bleed out about 1/2 of a master cylinder full at each wheel and call it a day. i usually use 1 to 1 1/2 quarts of fluid to do this.
one thing to remember is DOT 5 retains air like DOT retains water. i like to place the DOT 5 in the hot sun for a hour with the cap off to help solve this problem.

I knew I wouldn't get away with a two-letter answer for long!

OK,

No they won't "mix"; chemically that's not possible. Perhaps I'm splitting hairs, but that's where this thread is heading methinks.

Yes you could top up a non Dot5 system with Dot 5 but unless it's an emergency and nothing else is readily available, you shouldn't.

.....there, better?

Dave

To mix them is like putting milk in your beer. It won't kill you, but the mix is less than eithers normal quality.

Yes they are required by law to coexist without causing problems. The lowest boiling point fluid will control the boiling point, and as that's usually water in the hygroscopic glycol-based fluid. So cleaning out all old fluid makes sense from that standpoint.

Scare stories are perpetuated by the deceitful to scare the ignorant.

The major problem with the real DOT5 silicone fluid is that it entrains air and if you are not super careful you will end up with a spongy pedal. Yo have to be extra cautious to not shake the can or move it around and it should sit or days after being agitated. Rick Ehrenburg used silicone DOT5 in the Green Brick running the One Lap of America races, he would have noticed any problems with the stuff. I like it because it doesn't ruin paint. But I run mostly Wal-Mart DOT3 in my non-racer cars. So much for high tech!

R.

MrT2U's info is correct.We use Dot 5 in postal vehicles and have ZERO problems bleeding it.We bleed our brakes in the normal manner if using Dot 3/4 fluid.

In an emergency you can even use water.

What is the real question here. Do you want to blend 50/50 or are you worried about some trace amounts for a swap?

When Im worked at Hyster we had some models that used DOT 5. We got a letter from Hyster stating that their testing concluded that it was safe to mix DOT 3 and %, but it would turn into a really odd looking goo.

It does too.

Beautiful!

I had a similar plastic Coke bottle with DOT3&5 that I kept in my garage for several years with the intent of showing the same result - two liquids living peacefully very near each other, but not together. I was even using old used fluids, thinking that as a worst case scenario, used fluid would be it. After several years of periodic violent shaking, sitting open, closed, hot, cold, sun, dark... nothing but clear fluids. The tint in the DOT5 was fading a bit, and the DOT3 was still ugly, but that's about the only changes I saw.

One thing I feel I need to mention is the moisture absorbtion aspect of DOT5. Specifically, it doesn't, for all intents and purposes. That doesn't mean that water is going to pool up in the system somewhere because it doesn't get absorbed in to the fluid. DOT-not-5 will suck moisture out of the air, DOT5 won't. If you don't put water in your system on purpose, it's not going to get in there through DOT5 fluid. That's the primary thing that extends the life of DOT5 over not5. The other fluids absorb moisture from the air, reducing their boiling point, and leading to corrosion in the system. The not5 fluid needs to be flushed completely every couple of years at most. DOT5 could be flushed that often, but it doesn't need to be. I end up flushing my DOT5 about every 4-5 years, mostly because I keep upgrading the brakes or suspension. Been using the stuff since about 95, after my own unfortunate firewall paint experience.

Clair