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I've been googling like crazy and searched alfabb but can't seem to get a definitive answer on what to use. I keep finding arguments against each one. I've read people saying don't use Dot 4 for example because it dries up the rubber, and I've also read every one in Europe uses Dot 4. There doesn't seem to be any clarity, everyone seems to be using all three brake fluids although all three seem to be the wrong ones depending on the post you read.

I just got some stainless steel lines for my car and so want to drain out the brake fluid and install the new lines but am completely lost to what brake fluid to get 馃槶
 

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DOT 5 is synthetic. I read that it was developed for military vehicles that sat for long periods of time, then had to be ready to go on a moment's notice. It doesn't absorb water like DOT 3 and DOT 4, so moisture that gets it stays separate, falling to the bottom of the system. So you still get corrosion issues. DOT 5 does not have as much lubricity, which when I used it, felt like friction in the system. I didn't like it. Some folks swear by it. If you do use it you're supposed to flush the system and install all new seals. I didn't, and it was basically OK, but I wouldn't use it myself.

DOT 3 and DOT 4 are basically the same, DOT 4 having a higher boiling point. I use DOT 4. For street use, you're unlikely to get your fluid hot. For canyon carving or track use, yes. Regardless, these absorb water so flush every couple years.

Andrew
 

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To expand a little on what Andrew has written, all brake fluid now-a-days is labeled as "synthetic". This seems to be marketing more than anything else. DOT 5 is silicone based, DOT 3 & DOT 4 is gylcol based - the difference between 3 & 4 being the dry boiling points (DOT 4 is higher). DOT 3 & DOT 4 are compatible with each other but if mixed you should then assume the lower boiling point (BP). DOT 5 is NOT compatible with the gylcol fluids and should not be mixed. Then, to confuse the issue further, there is a DOT 5.1 brake fluid - which is glycol based (not silicone) but with an even higher BP.

I use DOT 4 in everything - including my vintage race MGA. Glycol based brake fluids are hygroscopic - they absorb moisture. As it absorbs moisture it the lower the BP. The key then is to flush the brake fluid every few years.

I've used DOT 5 in the past. It can be a little trickier to bleed as it seems to aerate with tiny bubbles. Re-bleeding after 2-3 days seems to help - I assume the tiny bubbles coalesce. Even though it is not hygroscopic it, too should also be flushed from time to time - perhaps at a longer interval. It has two disadvantages IMO - it costs a lot more and it can be difficult to find. DOT 4 costs a lot less and you can find it (or DOT 3 in an emergency) almost everywhere.

Back in the day, it was said that some brake fluids were not compatible with the rubber components in some European cars (my experience was with British cars). But that was decades ago - all the above brake fluids are compatible with the rubber compounds in today's cars.
 

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Sorry, I meant silicone for DOT 5, not synthetic.
There is also 5.1, whose makeup I don't know. I don't think it's silicone.
Andrew
 

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No, DOT 5.1 is glycol, just a very high temperature version. Really confusing choice of names there by the DOT folks.

I wouldn't use DOT 5 silicone in any street car. Stick with DOT 4.
 

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I started out as a 鈥減arts guy鈥 in 1973 at an Alfa dealer in Houston. Spent nearly 15 years at BAP/Geon, at one point the largest importer of imported car parts in the USA.

I use strictly Castrol DOT4 fluid. They used to call it 鈥淕TLMA鈥, which stood for 鈥渓ow moisture activity鈥, or so we were told.

Ate makes a good fluid, and Lockheed used to. Not sure if the latter is still extant.

I recommend the Castrol because it鈥檚 the right formula, and is easily found, meaning you won鈥檛 have any worries about mixing it with other stuff. Of course, all DOT3 and DOT4 fluids are supposed to be mixable with each other, and with other brands.

I buy mine by the gallon via Amazon prime, making it easy to refresh and replace my brake fluid when the mood strikes.
 

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For street cars I use DOT 4. I change it annually and usually use the Bendix 'blue" fluid, as it has the benefit of changing colour to brown as/if the fluid picks up water in use.
 
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