DOT 3 vs. DOT 4 brake fluid:
 

I’ve been meaning to ask this for a while, why do they even make DOT 3 and why do so many vehicles specify it? It’s basically the same as DOT 4 but with a lower boiling point, correct? I always use DOT 4 where 3 is specified, am I doing it wrong? :confused:

Thanks as always.

If you're doing it wrong, so did I...used DOT 4

Just a guess. "We've always done it that way. Why would we change to something new" If it ain't broke, I guess.

Isn't there one that you can't/shouldn't mix? I think it's DOT 5? Silicone based or something like that?

Interesting. Mixing Dot 3 and Dot 4 is actually not a good idea according to AI.

I’ve never contemplated using DOT 3 where DOT 4 is specified, only the other way around. I think that the issue is solved on this DOT 3 Toyota Tacoma I’m doing right now: :)
http://forums.pelicanparts.com/uploa...1752192950.jpg

I've been using dual rated DOT 3 and 4 for decades. I honestly have no idea why the industry still supports separate 3 and 4 ratings. You found the right stuff, Denis. Use it and never look back.

I have had vehicles that specify DOT 5. Yes, it is silicone, not glycol based. Can't mix it with 3 or 4. The whole idea was that it is not hydroscopic, so it will not attract water. My '76 Sportster specifies it, as did my 2000 Road King. I made the mistake years ago of putting the 3 and 4 into the Sportster to top it off and created one hell of a mess. It just gelled right up into little blobs, clogging the master and brake cylinders along with the brake line. It was a real job cleaning it all out.

Years later, after having replaced the entire hydraulic system on that front brake, I thought I could take advantage of everything new and just switch to 3 and 4. Hah! - Higgins, you idiot... turned out the materials used in the cylinders and line were not compatible with 3 and 4. Another hell of a mess to clean up, and now rubber parts to replace. So not only do the fluids not mix, the materials used in the brake cylinders, master and slave, along with the brake line, may not be cross compatible. Dot 5 is evil. Never should have been invented.

Dot 3 is slightly less hygroscopic so manufacturers can lower servicing costs by replacing it less frequently. Other than that, it is worse in every other way than DOT 4.

It is like fitting thread wear index 500 tires to a car. They last longer while having bad performance.

I guess if you're making a boring, light, economy car, stable over good is probably a good idea.

Dot 3 and 4 typically use EPDM elastomers. Dot 5 uses Viton (FKM).

This means the elastomers that work with Dot 3/4 aren’t compatible with Dot 5 and vice versa as Higgins pointed out.

I'd say that 1 car in a million EVER over the life of the vehicle gets it's brake fluid changed/flushed. Yes, folks here on the board probably do it fairly often (by comparison) and track folks. But my guess is that the avg Joe and Jane NEVER do it. The only time it gets done in most cars would be if there's a problem that causes the fluid to leak out, or if the fluid is bled enough that it's changed through bleeding. But I suspect most bleeding results in just some fluid being changed, not all of it.

Completely agree with your post here, most manufacturers have no recommendation for changing the brake fluid in their maintenance schedule and most cars never get it changed. I was surprised many years ago when I found out that BMW and Porsche had the procedure in their regular service interval. If DOT 3 attracts less moisture, that makes sense as far as why it's used in vehicles with no recommendation to change it.

I've always known that DOT 5 is completely incompatible with other fluids, I just never knew why. I thought that it was because it was synthetic but the stuff I bought yesterday is synthetic DOT 3/4. Apparently it's compatible with regular 3/4(?) I actually bought it by mistake, was dashing into NAPA to get it for friend's kid's old Tacoma. What is really strange is that I recently picked up a digital brake fluid tester online that checks moisture content. The brand new bottle pictured above of the Valvoline crap tests as having 1.5% water in it, I've never seen that in new fluid. It's not a crazy high percentage but all other brands test zero or close. :confused:

I’ve heard versions of this for years. All I have to offer is that I haven’t encountered any sort of hydraulic system failure in 50 years of using DOT 5 in 356s and my VWs. I use DOT 4 in my 911 and change it every year or before every track day.

Agreed...think I have an unopened one from Valvoline on my garage shelf. I wonder how long an unopened one is good for? Anyway, good heat rating on that brand..

Probably forever, more or less. Just get a cheap tester online, (everyone should have one), and test the bottle when you open it. I would place $$ on it being fine. Nothing else happens inside a sealed bottle of hydraulic fluid, other than possibly moisture. Which I doubt. It doesn't turn into wine or olive oil. :)

DOT5 is nice if you have an old car with ****ty Chinesium replacement parts. When the MC leaks - and they all do, thanks for that Winnie - your 5 figure paint job isn’t ruined in the engine bay.

Anyone remember the blue race fluid you can no longer get? Is the gold still available?

Eventually soaked through the soft, black knitted overfill line from my MC on my car turning it a shade of blue.

Motul...it is DOT4, was in two colors (blue & Gold) so mechanics could use alternate colors, letting them know when a flush was complete from old to new. I think the feds screwed this up by requiring all fluid to be gold...

It was ATE SuperBlue that was common and is now not for sale in the US thanks to some silly regs on fluid color.

ATE SuperGold is the OE street version of ATE SuperBlue racing fluid, with the exact same specifications in a traditional amber colored fluid.

Track guys used to like to switch back and forth when bleeding brakes as the color change was helpful to know when you were swapped.

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I still have some unopened cans of ATE Super Blue that is probably 100% fine. If you know how to properly change brake fluid, you don't need it colored to know when it's done. You syphon out and drain the old stuff before you even start. It's impossible to get all of the old out so you always have some mixing at first when you refill with new but it's in the calipers and quickly gets flushed out, once again, if you are doing it correctly.

If you are simply poring new fluid into a reservoir with old fluid in it, you don't know what you're doing.