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, Mon, 30 Apr 2007 17:27:42
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Silicone fluid....where do I begin??
Lot's and lot's of opinions on using silicone fluid (DOT 5) in place of DOT 3 or 4. I've done alot of research on this subject- talked to brake fluid manufacturers, brake specialists, brake designers, experts at Girling/Lockheed, just to name a few. So here's a short summary for you to consider.
1) DOT 3/4 absorbs water (hygroscopic). DOT 5 (silicone) does not. So, many people react to this by saying DOT 3/4 BAD, silicone GOOD. Not so fast! See, small amounts of water get into your brake system through seals, flex lines, and the reservoir. What happens to that water??? DOT 3/4 is DESIGNED to absorb small amounts of water (hygroscopic). DOT 5 will NOT absorb it. With DOT 5, these small water droplets congregate, often in the lowest point in your system and RUST/CORRODE that area. Or, they can VAPORIZE due to brake temp rise and you got NO brakes. Bad day.
2) Some people will tell you that DOT 5 won't properly swell older seals and the system will leak. Girling/Lockheed experts disagree. Their seals are not designed to swell with ANY fluid. If you had to wait for them to swell, then you'd have leaky brakes when you first rebuilt our system.
3) Some people will tell you that DOT 5 has worse temp rating than DOT 3/4. Not true. Look it up. DOT 5 has a better temp rating.
4) ALL car manufacturers are totally against DOT 5 for anti-lock brake systems.
5) DOT 3/4 and DOT 5 do not mix! A rotten congealed mess will result, sort of like the meat loaf in your elementary school cafeteria.
6) People that rebuild brake systems for old cars (like VSaabs) will warn you away from DOT 5. They will tell you that its more trouble than its worth, and that systems with DOT 5 will leak. They aren't making it up! This is most often due to people not replacing every single rubber and seal in the system AND purging/flushing the lines when changing to DOT 5. Traces of DOT 3/4 will cause problems over time in DOT 5 systems (see #5 above).
7) DOT 5 will not destroy paint like DOT 3/4 will. Very true. Yes indeed.
8) Many people claim DOT 5 will give a softer pedal. Reason- DOT 5 tends to trap small air bubbles when you add it in to your system- it loves to absorb air (actually, I do too). Very very difficult to get out. That (and poor bleeding) is the reason for the soft pedal with DOT 5.
So what do you do? I've got two Sonetts, one with DOT 5 for about 10 years- rebuilt entire system, flushed lines, NO problems. The other I just rebuilt the entire brake system and went with good old Castrol LMA DOT 4. Mainly for reason 1 above.
Final advice:
Change your brake fluid yearly or every two years max. If you use DOT 3/4, then it's absorbing moisture all the time (as it should!) and it can only do that up to a point. Brake temp performance drops with moisture absorption. If you use DOT 5, water droplets are pooling somewhere in the system and you better clean them out!
Eric in Vermont
PS- I got an article in the works on this topic with much more detail...
posted by 68.142.6...
, Mon, 30 Apr 2007 17:39:19, Mon, 30 Apr 2007 17:27:42
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