Silcone brake fluid

[Dale S.]

I have a 67 C-2. I bought ss sleeved calipers for it and at the time of installing them, I found a brand new 509 master cylinder. These were installed after cleaning out the brake lines & proportioning valve. I installed DOT 5 silicone brake fluid. This was about 10 to 15 years ago. I opened the master cylinder cap to install the correct cap( you probably saw Jerry's post). The bottom of the master cylinder had round spots of rust in the brake fluid (I live in N.M. so we have little humidity). I was shocked as years ago we changed all the brake fluid in the Air National Guards ground equipment and vehicles to prevent this. Does anyone have an idea what happened? Dale

[Dick W.]

Re: Silcone brake fluid

This is one of the problems with silicone fluid. It will not absorb water and therefore you have small droplets accumulating. Instead of having rust distributed throughout the system, you have pockets of rust. DOT 3 and 4 are mineral spirit based and are hydroscopic? (SP) (have an affinity for moisture). This is the reason that you will the system somewhat evenly corroded throughout.

I am sure that one of the engineers that frequent the board can give you a better description, but that is what happens in a "nut shell"

[Duke W.]

Re: Silcone brake fluid

Yes the term is hydroscopic - affinity for water.

Water is not miscible with silicone brake fluid, so whatever enters the system will stay separated and since silicone fluid has a higher specific gravity than water, the water will normally accumulate in high spots.

Switching to silicon fluid does not mean that you NEVER change the fluid, but you can probably go two to three times longer than the two year service life of conventional glycol brake fluid.

Duke

[Dale S.]

Re: Silcone brake fluid

Hi, I know what hydroscopic is, as Methanol and ethanol are also.(Racing fuels) I did not realize that Dot 5 was also. I dont think that the stainless steel calipers should be hurt, I dont know about the lines. But I am wondering about the preportioning valve and other pars of the system. Does anyone have any ideas on that? I am leaning towards draining all the Dot 5 and going back to regular breake fluid. I had a 1980 Olds wagon that I put 440K on, in 19 years and never completely drained the brake fluid with no problems. Any ideas on damage that could have occured would be appreciated. Thank you Dale

[Duke W.]

Re: Silcone brake fluid

DOT 5 silicone brake fluid is NOT hydroscopic.

Any water that gets into the system will remain separated and cause local corrosion.

If you have no leaks, I would not be concerned, but you should flush out the system will fresh silicone fluid. The flushing should carry out any globs of water.

If you want to go back to glycol fluid, I would only recommend completely disassemblling, cleaning, and rebuilding starting with everything clean and dry.

You cannot successfully flush out all traces of silicone fluid with glycol and vice versa. The only way to switch fluids is to start from scratch with everything clean and dry.

Duke

[Clare Carpenter]

DOT 3, 4 or 5 ? newly done restoration

What are the advantages or disadvantages in using DOT 3 or 4 vs DOT 5 silicon brake fluid in a newly restored chassis? Drum brakes, master cylinder and wheel cylinders all resleeved in SS. I haven't decided on whether to use SS lines or tin/steel. It seems the various vendors are all touting the advantages of silicon fluid for infrequently driven vehicles but as this thread points out, you can still suffer corrosion problems, just more localized. My understanding is that silicon is better on the race track, being more heat resistant but no good at altitude, with a loss of pedal. Since silicon offers no real protection against corrosion, is potentially dangerous if driving at higher altitudes, and I don't need the heat benefits, why use it? Isn't conventional fluid, with frequent changes, the way to go for most of us?. Am I correct in my reasoning?

[Duke W.]

Re: DOT 3, 4 or 5 ? newly done restoration

Conventional brake fluid absorbs moisture continuously. If you open the cap it will absorb moisture from the air, and if you drive in the rain, some moisture will probably be absorbed through the caliper piston seals. Absorbed moisture will mix homogeneously throughout the fluid and can cause corrosion throughout the system. Absorbed moisture also reduces the fluid boiling point. Like glycol based antifreeze, glycol based brake fluid has corrosion inhibitors that help prevent corrosion, but they are consumed as water is absorbed, so the fluid should be changed periodically, and most OEs recommend two years for normal driving.

Silicone fluid has no affinity for moisure so it will not absorb moisture from the air, but it can get in through the caliper seals in wet weather, and since water and silicone fluid are not miscible, the water will remain separated and cause local corrosion.

The only time to switch fluids if is you start with everything clean and dry, but hydraulic components are usually assembled with glycol fluid, so they must be disassebled, thoroughly cleaned, and reassembled with silicone fluid.

If you do start with everything clean and dry, I am biased toward using silicone fluid for a vintage Corvette that will see only occassional service and not normally be driven in the rain. Silicone fluid reduces maintenance by extending fluid changes - say six to eight years. I think some make the mistake that silicone NEVER needs to be changed, but this is not correct. It needs to be changed, but not as often as glycol fluid.

Silicone fluid tends to trap more air, so it should be pored in slowly. Air entrapment might be the source of the "brake loss at high altitude" stories, but it think that's human error.

Silicone fluid should definitely NOT be used on ABS brakes, since it has a tendency to cavitate when the valves pulse at high frequency.

So there are arguments on both sides, and if you are taking the system completely down you have a decision to make on what type of fluid to use. My analysis is that the reduced fluid change maintenance of silicone fluid tips the scales in that direction. Just remember that silicone fluid does not ELIMINATE fluid changes, just reduces the frequency of fluid changes.

Duke

[Terry M.]

Re: DOT 3, 4 or 5 ? newly done restoration

An additional point:
DOT 3 or 4 are a disaster if spilled on a painted finish. They are excellent paint removers. The risk of spillage is the reason some folks dislike pressure bleeders with these fluids.

DOT 5 will not damage paint, although its residue will make refinishing difficult without a thorough cleaning.

[Randy S.]

Re: DOT 3, 4 or 5 ? newly done restoration

I chose Dot 4 Castrol LMA. LMA stands for Low Moisture Absorbtion; may be marketing hype but I fell for it. But I still plan to flush system every 2 years as per Duke's recommendations.

Randy

[Duke W.]

Re: DOT 3, 4 or 5 ? newly done restoration

It's my understanding that DOT 4 fluids have a better corrosion inhibitor package and DOT 3 types, but I'm not sure about this.

I also use DOT 4 in all my cars that have glycol fluid - usually Castrol LMA, but Vavoline if Castrol LMA isn't on the shelf, and I flush them all every two years, except the SWC, which has silicone brake fluid.

DOT 4 is at least 2.5 times more expensive - about five bucks a quart, but I think it's worth it. When I upgraded the brakes on my Cosworth Vega (installed Monza vented rotors with their companion bigger piston calipers and a vacuum booster) in 1981 I found corrosion in the rear cylinders, so I replaced them, and that's when I started doing bienniel fluid changes on all my cars. I have not had to rebuild a hydraulic component since then, and it's now about 25 years since I've done anything to the CV brakes other than bienniel fluid changes and front brake pads - six sets; 4000 miles of race track hot laps tend to consume front brake pads at a rapid clip, but the rear drum linings are original.

Duke

[Chas Henderson #28127]

Re: Go from 3 to 4?

Duke,

Can you drain 3 and but 4 in? Can you mix them - as in srarting to drain 3 then adding 4 to the resivoir - to keep the master cylinder from going empty, and keep going with 4?

Chasman

[Duke W.]

Re: Go from 3 to 4?

DOT 3 and DOT 4 are fully compatible and one can be used to replace the other. They're both polyglycol based, but DOT 4 is blended for higher wet and dry boiling points and may have a better corrosion inhibitor package.

DOT 5 silicone-based brake fluid is NOT compatible with either DOT 3 or 4 and should never be mixed with either one.

Duke