DOT 3 in place of DOT 4 fluid in a pinch?

[Ask10]

Hi all, (sorry if this is the wrong category in advance)

Long story short, I'm in a pinch and DOT 4 is nowhere to be found. Just trust me on this one, there's none available where I am.

Anyway, I managed to repair the leak (95' Mercedes W140 diesel), but wanted to make sure I could fill it up with DOT 3 so I could get back on the road again.

-I know that DOT 4 has a higher boiling point etc etc, the main thing I need to know is if DOT 3 wont cause any damage to brake system in the meantime. I heard DOT 5 is completely incompatible, but wasn't sure about DOT 3 IN PLACE of DOT 4.

Please let me know, thanks!

-Adam (Ask10)

 

[LandCruiser]

The manufacturer clearly feels the braking system generates enough heat to call for DOT4.

If you're comfortable with boiling the brake fluid and losing brake power... also the "wet" boiling point of DOT3 is even lower.

 

[KidVermicious]

They're compatible. DOT4 is a bunch better than DOT3. If you have to drive the truck right now, DOT3 will work but you'll want to drain and fill with DOT4 as soon as you can. If you can wait a couple days, get on Amazon and order up a bottle.

 

[Ask10]

^^Thank you, that's what I needed.

I know DOT 4 is called for by the manufacturer, I literally have no access to it though and am stranded without ANY brakes. I am just trying to get home and am out in the countryside. I know how not to cook brakes and such (even though it's a tank), I just need to have brakes for when I am on the freeway and you know, I need to push the brake pedal...

I'll get it filled, bled etc and be on my way then. Thanks!

 

[T4R2014]

I put DOT3 in my BMW by mistake.

I did some research. Most guys say that you really have to drive like a maniac in order to start boiling that fluid. If you are just driving around town then DOT3 will work fine. I have had that DOT3 for a month and have not noticed any difference. I will flush it out with DOT4 later but for the time being I think it should be fine.

 

[Ask10]

Just updating, ZERO problems with a complete DOT3 flush and fill. Made it home and actually am still driving with it. That's because I found something interesting here: (scroll down to moisture content vs. dry point).

Brake & Clutch Fluids

While I know there's a lot of "scare propaganda" out there, I've been reading about brake fluid and such and it seems like this chart is actually accurate (regarding the life of brake fluid and it's boiling point in general. (EDIT: there is a second chart that I found showing that many cars reach this 2% absorption of water in 12-18 months), so really, fresh DOT 3 is better going to be better than DOT 4 most of the time in a real world situation (since most people don't change their brake fluid that often, meaning every year). I'm not saying DOT 3 is better than DOT 4 at dry point or wet point, but DOT 4 left in a car for a year or two is going to already be below the boiling point of dry/new DOT 3. So if one flushes with DOT 3 every year (very fast when you're used to it), then your car is "better" compared to most road cars when it comes to brake fade. Of course though, I wouldn't recommend DOT 4 vs. DOT 3 (this method) in trucks and such, but just as a "rest assured" memo, seems how that so many people don't have problems with brake "failure"/vapor lock as often as to merit a complete "scare the s***" out of you response (just from my experiences at a shop), then I'd say it's safe to say that the margin of "error" is quite high for boiling temps and what not, so new DOT 3 shouldn't worry anyone much, not for a regular city runabout anyway. That's all I'm saying.

Anyway, just wanted to share in case anyone finds themselves in my situation.

 

[lelandr]

Well, yes & no. Most German cars are going to require DOT4 brake fluid with a 2-3 year flush inertial. There is a very simple reason for this. Kinetic energy, which the braking system converts to heat, is 0.5mv^2. That means that if you double the speed, you quadruple the kinetic energy, and the heat generated in slowing down.

Your Mercedes comes from a place with extensive sections of road where you can operate it at significantly higher speeds than here. In this case, you probably wouldn't pass the TÜV with DOT3 as it could cause a safety issue.

Tomping around town at low speeds, you will be fine with DOT3. However, you could boil the fluid with repeated stops from high speeds. Also, the use of fluid with a higher boiling point was in the design considerations of your braking system.