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Brake Fluid

This component is part of Brake System.


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I am very interested in the silicon brake fluid since I am replacing all the lines etc. during the restoration. Do you feel the silicon fluid is a big improvement performancewise over DOT 4 or 5? Or is the main advantage longevity of the fluid?

I would also be interested in Frank's input on this, as I struggled with this when I restored my breaking system. My restoration shop advised against it, believing that my reconditioned brakebooster and rubber components exposed to the synthetic fluid (silicon?) would not hold up and I would have problems. Any long-term users of synthetic fluid for brakes?

Frank: I use silicone fluid (DOT-5) because the brakes on my car faded when coming down an Alpine pass on its maiden voyage, then again a year later in an MBCA time trials. Therefore I consider it a handling enhancement.

Will: I have had both great and bad luck with synthetic brake fluid. I have used it in a 190 SL and my 280 SL. Both did just fine. I used it because it was cool and the in-thing. Both cars had new brake systems. The only used part on the 280 SL was the proportioning valve by the gas tank. I have also tried the synthetic on a customer's 190 SL and had the fluid pour right out of new wheel cylinders. What a mess. Changed the wheel cylinders and it happened again. Perhaps it was the fluid. I was told only Dow and GE make the stuff. The only reason to use synthetic that I can see is it will not mess up your paint. Otherwise, the brake system still collects contaminants such as moisture, dirt, and old rubber. Which means you have to bleed the system every few years to get clean fluid in there. I bet there is less possibility of the rubber flex hoses swelling up internally. That would be a plus for synthetic. The higher boiling point is not a factor unless you race your car. I did get a slightly spongy pedal with the synthetic, but got used to it.

I have been using silicone Brake Fluid (I.E.: DOT 5) for probably 20 years. I've only had a problem on one car, 70 Barracuda Convertible, that I just flushed the system and the Wheel seals leaked. This is because it does not swell the wheel seals. After installing new wheel cylinders it worked fine. You've overlooked one main advantage. Silicone Brake Fluid is hydrophobic, I.e.; it does NOT absorb water (thus I see no need for of annual flushing.) Standard DOT 3 brake fluid absorbs water from the atmosphere, breathing from the Master Cylinder lid. This is why the DOT 3 containers all state that it must be kept sealed. In my experience the main cause of failure of wheel cylinders on shoe brakes is the corrosion at the bottom of the cylinder from water droplets. I admit that disc brakes run at a higher temperature and are not as subject to this (my 67 230 SL still has shoe brakes on the back). But, although it will mix with DOT 3, there's no advantage unless you're rebuilding the system and everything is clean, without moisture. Silicone will not take the water out of an old system.

Someone recommended that since I will not be driving my car much during the winter I replace the brake fluid with silicon based brake fluid. The silicon based fluid apparently does not absorb water and so prevents problems with the brakes freezing up over the winter. Someone else told me not to use silicon based brake fluid because the water that will condense in the system is not absorbed and can result in losing braking in a panic stop situation.

I have been using Silicone brake fluid in my Saab since Oct.1987, and have not had any problems. I am in the process of re-doing my whole 113 brake system, and think i will go the silicone route here as well. Make sure you do a very thorough flush if you decide to go silicone - it doesn't mix too well with the normal type.

Have used silicon brake fluid for over 20 years and never had a bad caliper of brake cylinder since. Still have part of a gallon that I picked up via mid-night requisition from the Army in 1976.

Pete Lesler: when I restored a 250SL in 1983, I installed all new calipers and master cylinder as well as lines. I used silicon at that time and ran the car a numerous MBCA national driving events for probably 10 years with zero problems. I think I replaced the fluid once during those ten years. The only time I lost braking was when I replaced the silicon with DOT 4 and probably didn't get all of the silicon out. I never had a problem with the silicon brake fluid. I installed the silicon for one reason which was to prevent any paint removal if the fluid boiled out of the reservoir or accidentally spilled on the paint. Silicon is DOT 5 which means a higher boiling point, which is an added benefit if you intend to run time trials, or competitive rallies.

Thanks for your inputs on silicone brake fluid. A couple of you have used it with great satisfaction. I have tracked down a couple of other sources and the long and short of it seems to be:

  1. Unlike Glycol based fluids Silicone based (DOT 5) do not absorb water. But, water can still get into the system. And then it collects at the lowest point water being heavier. This water can cause corrosion in the areas where it collects.
  2. Silicone fluids are not compatible with Glycol so the system has to be thoroughly cleaned. A recommendation was to change over after a restoration when all of the old fluid is out of the system.
  3. Recommended for vintage cars that are seldom driven our stored for extended periods of time.

I guess that since I intend to drive my car on occasion even during the winter I would be just as well off to use a good brake fluid DOT4 and change it regularly.

I have been using silicone Brake Fluid (DOT 5) in all of my older cars (rebuilds) for over 30 years and only had one problem. I honed and rebuilt the wheel clyinders on our (now my son's) 1970 Baracuda Convertible, and they leaked. I replaced the wheel cylinders with new and no problem. However DO NOT use it in a newer car with ABS, I don't think the pump will take it.. Normal Brake Fluid (DOT 3) is hydroscopic and will absorb water from the air. You'll notice that old brake fluid gets thick and milky looking, This is due to the absorbed water. This is why most bottles have warnings to keep the bottle tightly capped. This is also why the service manuals suggests you flush out the old fluid periodically. I don't think this is necessary with DOT 5. I don't know if DOT 3 can absorb enough water to actually freeze up. Being an alcohol base, I don't think so. The main reason why I use DOT 5 is that it prevents rust pitting in the bottom of the brake cylinders. This is not as big a problem with disk brakes as it is with shoe brakes.
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