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Old 03-31-2003, 09:08 PM
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Post DOT 3 vs DOT 4

I am going to be flushing the brake system on my Jag. The shop that will do the work suggested I use DOT 4 fluid. Anyone know what the major differeances are and is there a problem if any of the old DOT fluid is left in the system

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Mark

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Old 04-01-2003, 10:05 AM
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DOT 4 is silicone based and won't absorb moisture. If you don't drive the car much it's a good idea. It you have DOT 3 in the lines, when the DOT 4 and DOT 3 mix the fluid will gell and lock the brakes. All of one must be flushed out (usually with alocohol) before putting the other in -- THEY DO NOT MIX! I ran DOT 4 for a while in a daily driver. Accidentally poured a couple ounces of DOT 3 in, realized my mistake, and quickly used a large syringe to empty the reservoir. Since I hadn't moved the car and it hadn't been in long, I thought I would be fine. But the left front brake started sticking the next day. A little of the fluid had got in the line, so I had to drain and flush the whole system. DOT 3 went back in.

DOT 4 will also expand more with heat. Got the bright idea to use it in my motorcycle. Bike front calipers use very little fluid to clamp. Park bike in parking lot on hot day, go to leve from work and the front calipers are locked! Had to bleed them to drive home. With the car this was never a problem as the calipers aren't in direct sunlight.

After all this, I'd only use it if a car sits for long periods so the brake system doesn't corrode. Pedal feel is about the same, but may be just a bit "mushy" with the DOT 4 because it will compress just a little. Not much, some people don't notice the slightly different pedal feel.
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Old 04-01-2003, 10:25 AM
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Thanks farna! I never asked the question but this is the most 'to the point' info I have seen on DOT4- I appreciate your response to Jag Daddy!!! Thanks again!
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Old 04-01-2003, 11:35 AM
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found this page looked interesting, let me know what you think http://www.synlube.com/brake.htm

Thanks
Mark
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Old 04-01-2003, 12:16 PM
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I'm not in the shop right now, I will walk out and check in a minute, but I was thinking that DOT3 & 4 were compatable with the 4 having a higher boiling temp. DOT 5 being the silicone based stuff, while it doesn't absorb moisture it will absorb air as it sits over periods. I don't know what year Jag, but we used to use Girling fluid exclusively in them because the fluid at the time would swell the rubbers in the hydraulic systems. It may have been that the Girling was more like the later DOT fluids, but when have the English been that on the leading edge of technology, not discounting Colin Chapman and Westlake.
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Old 04-01-2003, 01:38 PM
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Woodz is right, DOT 5 is silicone based. DOT 3&4 are compatible with DOT 4 having a higher boiling point.

Do a search and you'll find the thread where this subject was discussed at length.
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Old 04-01-2003, 02:36 PM
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I just got back in and as 4 jaw chuck (you must be or were at one time a machinist) confirmed 3&4 are compatable with 5 being silcone, I also found several synthetics that are now affered that have the high boiling and non corrosive elements of silcone with out the air absorbtion problems.
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Old 04-01-2003, 03:57 PM
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I would not use the DOT 5 unless you only drive your ride a few times a year, people seem to have some braking problems with the DOT5 fluids, me i always had a spongy pedal, till i flushed it out and replaced with Wagner Dot4.
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Old 04-03-2003, 05:39 PM
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i work for a chevy dealer and i'll have to say that dot 3 and 4 are compatible. dot 4 does just have a higher boiling temp. dot 5 however is silicone based and should not be used. if you see any contaminants though you should completely flush the entire system. it's always better to use one type of fluid no matter witch job your doing but if you can't get all the dot three out you will be ok.
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Old 04-03-2003, 08:09 PM
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A mix of 3 and 4 works from what Ive seen. Have a look at THIS.
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Old 04-04-2003, 03:01 AM
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The spongy feel Roys63 described is the effects of silicone absorbing air over time, that's why you should avoid the silicone based types. There are DOT 5 ( that's a standard, and initially only the silicone fluid fluid could meet it) that are NOT silicone based so they don't have that problem, but they stil resist the corrosion that is common in standard fluids. I can give several suppliers, but I get mine from Tri-R Spectro oils, they supply oils and chemicals to the motorcycle industry, but there are others as well.
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Old 04-04-2003, 04:49 AM
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Can someone tell me which is better to use in a system #3 or #4?
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Old 04-04-2003, 07:31 AM
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DOT 3 fluid is the standard brake fluid we've had since the earliest days of hydraulic brakes. It is used in virtually every car made that does not have ABS brake system.
DOT 4 fluid is used in most modern cars which have ABS systems or 4-wheel disc brakes. It has higher boiling point and anti-foaming agents.
(It is compatible with the earlier systems but no brake fluids should be mixed, use either 3 or 4 but not both!)
DOT 4 is the controversial SILICONE fluid. Many folks with early cars have tried it and been unhappy due to the phenomenon of "squishy" pedal.
I would only use it in a dead car to protect the brake system components as it does not "draw" moisture. Then flush, refill with 3 or 4, and drive.
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Old 04-04-2003, 08:58 AM
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Thanks for all the info guys, I'm going to look into some of the synthetic stuff. How much does a system typically hold and how much do I need to flush the system. Should I do a total flush with cheap DOT3 and then purge the system again then add the new stuff?

Thanks
Mark
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Old 04-04-2003, 02:26 PM
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You could do that to cleanse any debris, but you will still need to flush with the synthetic again to help remove the traces of the other. I would recommend a gallon so you can flush, fill, bleed and have some for later. Speedway motors offers a synthetic DOT5 and they aren't far, but a decent parts store should have or be able to get it over night.

[ April 04, 2003: Message edited by: woodz428 ]</p>
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