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Silicone Brake Fluid Pro and Con

1625 Views 5 Replies 4 Participants Last post by  beechkid
Looking for input on the use of Silicone brake fluid in older Mustang (67 GTA). I replacing my second master cylinder in 2 years. My cars sits alot and I'm guessing this is what has caused the deterioration of the current master cylinder. I have a new (rebuilt) Master cyl. but before installing I'm considering use of DOT 5 silicone fluid. I know the benefits but am concerned about leakage of old wheel cylinders, calipers, lines using the new fluid.

I used silicone fluid on my 86 show truck for several years with no ill effects (it sat alot as well). I didn't even bother to clean out the lines (not recommended I know). However it was a very low mileage, newer vehicle.

Anyone have any input?

67 GTA S code fastback
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For a car that sits a lot I would recommend silicon brake fluid for it's non-corrosiveness. Glycol fluid will absorb moisture as where silicon won't. However I know if you replace your regular brake fluid about every 2 years this shouldn't be a problem, unless you live somewhere that is very humid.

Besides cost, silicone fluid can have it's drawbacks. It tends to compress under high temps which gives the brake pedel a spongy feel. Personally, I would go with silicone fluid for it's non-corrosive traits.

See my 69 convertible and the VMF parking lot at:
Although silicone fluid won't absorb moisture, this doesn't mean it will eliminate it. Because water won't mix, it will be trapped in low spots in the lines and calipers/wheel cylinders. I would stick with DOT3/DOT4 fluid changed every 2 years and only added from a sealed container. I'd only use DOT5 in racing applications for the higher boiling point and then only when compatible with rubbers and plastics in the rest of the brake system. IMHO only though.

'66 A-code Fastback (therapy)
'89 Town Car (SWMBO D'driver)
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In theory DOT-designated fluids are backwards compatible and mixable. I.e. you can use, e.g., DOT-4 in DOT-3 apps, but not the other way around.
DOT-5 can be used in place of (and even mixed with) DOT-3/4.

Mixing silicone DOT-5 with a lower DOT rating will, however, eliminate the non-hygroscopic properties of silicone DOT-5. So if you plan to switch to DOT-5, be sure to remove all traces of the old brake fluid from the lines and cylinders.
I'm glad you mentioned mixing being a no-no with silicone fluid....

The systems I've built for race cars have never even seen anything other than silicone fluid and, if retrofitting, I recommend using all new seals, cleaning the hard parts throroughly and flushing the system with hot silicone fluid prior to filling and bleeding. I also recommend vacuum bleeding, if possible, with the bulk supply coming directly from a factory container of silicone fluid, with connection hoses (both for bleeding and flushing) being new or only used with silicone...

For everyone short of the autobahn types and race cars, regular DOT 4 fluid should work fine....

I have used DOT 5 for about 20 years in my cars. You still have to flush the system periodically because it will collect moisture in the master cylinder. However I have never lost a seal yet using this stuff. However, they now make a DOT 5.1 which is not silicone but has many of the same positive charateristics with out the negatives. If I had to do it again right now, I would use DOT 5.1.

BTW, DOT 5 does not create a spongy pedal feeling. It has a higher boiling point than DOT 3 and the DOT 4 of years ago. Where the problem comes in, is once it boils, it must cool far greater than DOT 3 or 4 to "restore itself" to regain total function.

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