Brake Fluid-Dot 3 vs Silicone

[Tom C.]

I have 4 questions.

1. What is the current designation for GM Supreme 11 brake fluid?

2. Does anyone know of a substitute which they think is better?

3. What is the problem with silicone brake fluid? (i have a 66 coupe with SS sleeves)

4. What size fitting wrench do I need to work on my line fittings and bleeders?

Thanks Guys.

[Joe L.]

Re: Brake Fluid-Dot 3 vs Silicone

Tom----

1. GM Delco Supreme brake fluid is a glycol-based, DOT 3 fluid.

2. The GM DOT 3 fluid is excellent for a glycol-based fluid. Castrol DOT 4 fluid might provide some benefits and it is totally compatible with DOT 3 fluid, so you don't have to scrupulously clean the system. However, if I were using glycol fluid, I'd probably go with the GM fluid. At the risk of being labeled a heretic, I'll also tell you that Ford-branded DOT 3 fluid has a VERY excellent reputation. In fact, I'm told by reputable sources that it is used by all of the NASCAR teams, including the "GM Goodwrench" team(with tape over the container labels). You can pick this up at your friendly Ford dealer, if you so choose.

3. There are many benefits to Silicone DOT 5 fluid, and that's what I use in my 1969. First off, it is non-hygroscopic, so it doesn't absorb water. Second, since it is non-organic, it doesn't decompose or biologically degrade, and it won't destroy paint if it's accidentally spilled on it. There are many downsides, too. Most important is the fact that you have to completely clean your brake system of the glycol fluid before you install silicone-based fluid. That means complete removal and disassembly of the calipers and master cylinders. Flushing of the steel brake lines with silicone fluid and replacement of the rubber brake hoses is also highly recommended. So, it's not really practical to convert, except at time of complete system overhaul. There is much more discussion and information in the archives on this; others and myself have posted many times before on this one.

4. Brake line fittings require a 3/8" TUBING WRENCH; most bleeder valves, if of stock configuration, also require a 3/8" wrench size. Rubber brake hose fittings require a 5/8" wrench size.

[Joe L.]

Re: Brake Fluid-Dot 3 vs Silicone

Tom----

1. GM Delco Supreme brake fluid is a glycol-based, DOT 3 fluid.

2. The GM DOT 3 fluid is excellent for a glycol-based fluid. Castrol DOT 4 fluid might provide some benefits and it is totally compatible with DOT 3 fluid, so you don't have to scrupulously clean the system. However, if I were using glycol fluid, I'd probably go with the GM fluid. At the risk of being labeled a heretic, I'll also tell you that Ford-branded DOT 3 fluid has a VERY excellent reputation. In fact, I'm told by reputable sources that it is used by all of the NASCAR teams, including the "GM Goodwrench" team(with tape over the container labels). You can pick this up at your friendly Ford dealer, if you so choose.

3. There are many benefits to Silicone DOT 5 fluid, and that's what I use in my 1969. First off, it is non-hygroscopic, so it doesn't absorb water. Second, since it is non-organic, it doesn't decompose or biologically degrade, and it won't destroy paint if it's accidentally spilled on it. There are many downsides, too. Most important is the fact that you have to completely clean your brake system of the glycol fluid before you install silicone-based fluid. That means complete removal and disassembly of the calipers and master cylinders. Flushing of the steel brake lines with silicone fluid and replacement of the rubber brake hoses is also highly recommended. So, it's not really practical to convert, except at time of complete system overhaul. There is much more discussion and information in the archives on this; others and myself have posted many times before on this one.

4. Brake line fittings require a 3/8" TUBING WRENCH; most bleeder valves, if of stock configuration, also require a 3/8" wrench size. Rubber brake hose fittings require a 5/8" wrench size.