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Old 04-30-2003, 06:12 AM
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Post Regretting using silicone fluid - advice?

I have used DOT5 silicone fluid in the new brake system of my 32 but wondering now if I shouldn't have.
I read it has got smaller particles which leak more - and I now have a weep in one (new) rear cylinder (9" Ford) and I can't stop a stainless hard-line where it meets a thru-bulkhead fitting from weeping a tiny amount under pressure. I'm wondering whether I should just flush it and use DOT4, my questions are:
* Is a miniscule leak with silicone likely to seal OK with DOT4?
* I've read that I should use alcohol to flush the system - what sort & where might I get it?
* would the silicone have affected the seals? -ie would I need to change the rear cylinder seals to be fully compatible with the new fluid?

One of my reasons for choosing silicone was to prevent paint damage from spills/leaks, but maybe I've created leaks using it??
ANy advice? thanks, Limey

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Old 04-30-2003, 06:24 AM
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[quote]Is a miniscule leak with silicone likely to seal OK with DOT4?<hr></blockquote>

Probably not, but DOT 4 does have better seal swelling properties so your slave cylinder leak may improve...I wouldn't count on it however.

[quote]I've read that I should use alcohol to flush the system - what sort & where might I get it?<hr></blockquote>

Regular Isopropyl will work but you should use a higher purity alchohol such as 95% Methanol, you will have to disassemble all your brake components to remove every single trace of DOT5 fluid. Simple flushing is only appropriate for the lines.

[quote]would the silicone have affected the seals? -ie would I need to change the rear cylinder seals to be fully compatible with the new fluid?<hr></blockquote>

Your old seals will be fine.

“She may not look like much, but she's got it where it counts, kid. I've made a lot of special modifications myself.”

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Old 04-30-2003, 06:32 AM
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Go to Brake article and read the excellent article by Dean Oshiro.

To quote from part of the article:

"Brake Fluid: Brake fluid is the liquid that transmits the force through pressure for the brake pedal to the brake lines. Basically the brake fluid does not compress so it transmits this force (pressure) without lost.

One of the worse enemy of brake fluid is heat. If the brake fluid boils or there is a leak in your system there will be a lost of this incompressibility and your pedal travel will increase. Not all brake fluids are the same. Most brake fluid has ethylene glycol as it main ingredient. Ethylene glycol has lubricating capability for the rubber parts and has a high boiling point. Moisture is another enemy of brake fluids. All bake fluids will absorb moisture form the atmosphere, this moisture lowers the boiling point of the fluid drastically. This moisture also can effect the balance of the system casing corrosion. A perfect example of moisture getting your system is the early Corvette brakes where it was common to change the calipers or a regular basis due to contamination and corrosion.

Silicone brake fluid has a higher boiling point (around 700 degrees F.) than the ethylene glycol base fluids, but the major disadvantages is not "hygroscopic". Hygroscopic? "Altered by the absorption of moisture" What this means is since it is not a glycol based, when moisture enters the system it is not absorbed by the fluid. This results in beads of moisture moving through the brake line, collecting in the calipers. Since it is not uncommon to have temperatures in excess of 212 degrees F. (the boiling point of water), this collection of moisture will boil causing steam and vapor lock, this in turn will cause system failure. Silicone (DOT 5) is also highly compressible due to aeration and foaming under normal braking conditions.

If you are changing from a glycol base fluid to silicone or the other way around. The two types do not mix so your system should be completely purged, disassembled and dried out. When the two fluids are mixed you will get a gummy substance and it will really mess up your system.

We recommend using a good DOT 3 fluid. Wilwood makes a hi-temp fluid with a minimum dry-boiling point of 570 degrees F Dry-boiling point is measure in its virgin non-contaminated state. Wet-boiling point is the temperature a brake fluid will boil after it is fully saturated with moisture. DOT 3 fluids have a minimum wet boiling point of 284 degrees F.

Brake fluid should be changed periodically due to contamination. Never mix different DOT brake fluids. Under racing condition you would change these fluids like changing your oil.

Copyright reserved by Dean Oshiro. Reproduction without written approval is a violation of Copyright Laws. Sept 2000 "
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I've always wondered why the rubber in brake systems is so delicate - look at it wrong and it deteriorates, while the rubber in an automatic transmission can be abused beyond reason with nasty oils and a lot of heat and still keeps truckin' along.

Remind me of the time my brother-in-law as in college and had a part time job doing the grave yard shift at a service station. Seems a customer needed his brake master cylinder topped off so Tom did the service with an unmarked can of stuff that was sitting on the counter and he thought was brake fluid. Next morning he asked the station owner what was in the can, oner relpied, "Automatic transmission fluid".
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