Brake Fluid

[nucoc]

Hi all.
What type or grade of brake fluid should I use in my 1975 SL?
Roy

[MikeSimon]

Use DOT4. Originally your car probably had DOT3. DOT4 is compliant with DOT3 and has a better high temperature performance. DO NOT use DOT5!!!

[Rolf-Dieter ✝︎]

This should be fine to use.

====> https://www.carid.com/1975-mercedes-s-class-oils-fluids-lubricants/ate-sl-dot-4-brake-fluid-4393300816.html

Dieter

PS or check on Google

[Benz Dr.]

Use DOT4. Originally your car probably had DOT3. DOT4 is compliant with DOT3 and has a better high temperature performance. DO NOT use DOT5!!!

Not to disagree, but why no DOT 5?

[WRe]

Hi,
see ATE product video "ATE brake fluids and how to make the right choice": https://youtu.be/eSxZKAn1fts (in German with english subtitles).

"When choosing the brake fluid the important thing is to observe the manufacturer's requirements. You'll find instructions on the filler cap of the expansion reservoir."

BUT:  On our filler caps you see "Nur ATE blaue Original Bremsflüssigkeit verwenden", which means "Use ATE blue original brake fluid." At that time, however, one did not mean the color of the fluid, but the color of the canister in which the brake fluid was located.
Today blue brake fluid is on silicone basis which you should not use otherwise you clean the whole brake system and remove the old fluid completely. Use glycol-based brake fluid which is yellow/amber or clear.

Mixing DOT-3 and DOT-4 brake fluid is possible, but generally the brake fluid that is specified on the reservoir cap or is approved by the vehicle manufacturer should be added to the brake system.
In order to obtain a brake fluid compatible with DOT 3 and DOT 4 with DOT 5 specifications, the glycol-based brake fluid DOT 5.1 was developed. DOT 5 (silicone based) brake fluid must not be mixed with any other type of brake fluid. However, it is not a problem to mix DOT 3 or DOT 4 with DOT 5.1, although this changes the boiling points.
...WRe

[ejboyd5]

Only the US DOT could have thought it a good idea to call an improved version of DOT 3 or Dot 4 by the name DOT 5.1, which leads everyone to believe it is a version of the silicone based DOT 5 fluid.  Wouldn't it have been only common sense to call it DOT 6 rather than using the trendy, ".1," thereby limiting the possibility of confusion between the two base types?

[MikeSimon]

When our SLs were produced, DOT3 was generally common. I don't know when DOT4 was first introduced. Only difference between 3 and 4 is a highe boiling point of 4, thus a better performance when hard and often braking is used. Stay away from DOT 5.

[WRe]

Hi,
there are some differences between DOT 3 and DOT 4/5.1 (see att.), but mainly DOT4 and 5.1 contain boric acid ester/borate ester which can attack rubber in your braking system.
If you look into the maintenance manuals of those years you'll find a instruction to use brake fluids  corresponding to 331.0 which leads today to A0009895605 a DOT 4 plus brake fluid, whatever this is.
In other old documents I found a recommendation for ATE type S with a boiling point of 290°C. But there is no such ATE or other braking fluid.
It's confusing but with DOT 3 or DOT 4 you will be on the right side.
...WRe

[Benz Dr.]

No one has explained why you shouldn't use DOT5. I would use it in a hydraulic clutch system; would I be wrong to do that?

I could see using it if I was starting with all new parts that never had any regular brake fluid near them.

[fin230sl]

Also wondering why not DOT5…? Obviously do not mix it with other DOT’s but for a new or totally flushed system, why not? My understanding is that one main benefit with DOT5 is that it doesn’t absorb moisture and hence would be a good candidate for a classic car. Am I missing something?

[UJJ]

I changed to DOT5 10 years ago after a totally flushed system. No issues.

[Richard S]

Since I live in a very damp climate area, I switched to DOT 5 about 20 years ago.  The only issue is that the brakes have a slightly softer feel.  I still bleed the brakes every 10 years or so just to check the condition of the fluid.

[Benz Dr.]

DOT 5 won't absorb moisture so any water that collects in the fluid will sit in a low area and could rust through a brake line over time. Also, silicone won't produce any white smoke if you have a failed master cylinder but that shouldn't be a problem on our cars since the vacuum port on the booster is placed higher than the MC. It would be a problem on a 190SL where vacuum is drawn from the bottom of the booster. 

So, I don't see any reason why you can't use it as long as it's used properly. DOT 4 will work just fine but should still be flushed out every couple of years.

[mdsalemi]

No one has explained why you shouldn't use DOT5. I would use it in a hydraulic clutch system; would I be wrong to do that?
I could see using it if I was starting with all new parts that never had any regular brake fluid near them.

You asked? Here's some information from other forums and websites...

https://www.dandyclassics.com/dot-5-remvloeistof-oldtimer/

Other thoughts on the matter:

"DOT 5 is silicone-based and does NOT absorb moisture. That's a good thing as far as the deterioration of the fluid goes. It's a bad thing for cars driven infrequently and/or when the system is not flushed immediately before storage as the moisture that DOES enter the system will head for the low spots and stay there...Instead of having some moisture spread out all throughout your fluid you'll have pockets of pure water sitting in wheel cylinders and calipers, or loops in your lines. DOT 5 is great for race cars because of its VERY high boiling point but you won't catch me running it in a street car..."

"DOT 5 doesn’t absorb moisture, so some folks think it is the better choice, but is it?   Before you go rushing off to the parts counter, remember even though DOT 5 doesn’t absorb water, it can’t/won’t prevent moisture from entering the brake system.  And since the water isn’t absorbed by DOT 5, moisture puddles and causes localized corrosion within the brake system.  As funny as it sounds, DOT 3, 4, and 5.1 absorb moisture, which in turn eliminates the puddling that can cause corrosion. More importantly, when brake fluid heats up, water trapped inside the brake lines (but not absorbed by the brake fluid) is converted from liquid to vapor.  Steam compresses easier than liquid.  With this in mind, imagine barreling down the road at high speed and hitting the brakes.  When the hydraulics sends DOT 5 fluid through a pocket of steam in the line, that drop in pressure creates a soft pedal..."

All that being said, I'm sure I have MB approved brake fluid in my brake system. Or do I? Last it was flushed was some years back. Probably time for a change when I get around to changing the master cylinder. My brakes work fine. I think I don't drive or brake hard enough to worry about boiling points! Right?

[MikeSimon]

Even if used in a totally new or thoroughly flushed system, care has to be taken that all soft (rubber) parts in the brake system are compatible with DT5 silicone fluid.
There are some vehicle manufacturers that used DOT5 in their new vehicle brake systems for a while but they have changed back since. Why?
I will not use DOT5 in any of my vehicles. The ONLY advantage of DOT5 in my book is, that it is not agressive to paint. Being careful helps.

[rwmastel]

BUT:  On our filler caps you see ....

When our SLs were produced, ....

Did everyone miss that the OP is asking about his 1975 SL??  Does that impact this long, detailed discussion?  Have we helped the OP?  Is everyone just having fun arguing?