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Discussion Starter #1
I'm installing a power brake conversion. The reservoir on top of the master cylinder states, "DOT 3 Fluid Only" Is this a statement from before the days of DOT 4 and 5 fluid? I plan on a complete flush of the system with the new fluid.....unless there's some reason I shouldn't do this. Thoughts?
 

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My understanding is that you should not because the rubber in older stuff is not compatible with silicone based brake fluid (which DOT 5 is). I would recommend (and I use) DOT 3 synthetic.
 

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If you have totally rebuilt the system - new hoses and everything the components they use now are compatible with the silicone fluids. DO NOT mix them though. You will have to flush the system then fill and bleed it top to bottom. The nice thing is you don't have to worry about the lines rusting from the inside out like DOT3 does over time since it take on water/condensation. Be sure to remark your reservoir cap if you do it! As reminder to you or any future owners.....
 

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Discussion Starter #4
Thanks for the responses!
 

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Planning on flushing my brakes real soon. My understanding is that DOT4 is not synthetic, just has a higher boiling temp, and therefor "better" than DOT 3. I don't want to use DOT5. Thought's on DOT4 going into my 1969 XR7 convertible cougar?? If so, any brand from Pep Boys or the like?
Thank you.
 

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I've had DOT5 in my cat since 1992. Many track days and lotsa miles. Won't peel paint like the DOT3-4. Some like it, some don't.
I happen to like DOT5.
 

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Planning on flushing my brakes real soon. My understanding is that DOT4 is not synthetic, just has a higher boiling temp, and therefor "better" than DOT 3. I don't want to use DOT5. Thought's on DOT4 going into my 1969 XR7 convertible cougar?? If so, any brand from Pep Boys or the like?
Thank you.
DOT 4 will work, wherever you buy it!
 

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I am running DOT 5 as well. So far I have had no issues because of the fluid. I put it in with a complete rebuild, which makes it much easier, I just flushed my hard lines out with laquer thinner and compressed air. I have even taken my front calipers back of and taken the pistons out and there were now ill effects on the rubber seals.
 

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What are the benifits of DOT 5? Since I am rebuilding/replacing everything, would I be better off going to DOT 5 on assembly?
 

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Here is one drawback ... you take it to a shop to service (I know we all do our own work, but ....) and they see the brake fluid level is down. Tag says DOT3, guess what gets added. Not good.
 

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What are the benifits of DOT 5? Since I am rebuilding/replacing everything, would I be better off going to DOT 5 on assembly?
Higher boiling point, doesn't attract moisture and won't hurt the paint. I think the seals last longer.
 

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So if I am starting fresh is the opinion I should go with DOT 5 and make sure 3 is never added?
 

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My 65 Mustang has had DOT 5 since the mid 80's. It's sat since 1996....it still has a good pedal. When I bought the DOT 5 years ago it came with a sticker to affix to the top of the master cylinder stating it was filled with DOT 5. Hope this helps.
 

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Still thinking. Sounds good though. Everything in my system will be brand new or freshly rebuilt. All the lines will be new as well as the wheel cylenders and the calipers will have new seals installed. Master brake cylender and booster will be rebuilt.
 

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No one mentioned:

Dot 3 firm pedal / Dot 5 spongy pedal due to higher compressibility

Dot 3 hydroscopic so they mix with water and still perform well even if boiling temp is lowered. That shouldn't matter to a passenger car.

Dot 5 won't mix with water so when water is present you get dot 5/water/dot 5/water/dot 5/water and that combo does decrease braking efficiency.

I use Dot 4 and change all cars every two years. With the large vacuum pump I have each car can be done in 15 minutes. Actually takes me more time getting to the bleeders on some of the front discs.
 

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If you want the higher boiling point get DOT 5.1. It's not silicone based and will damage painted surfaces. DOT 5 will give you a softer pedal than DOT 3, 4, or 5.1. You can order the DOT 5.1 on Amazon for $7.99 per 12 oz. bottle.
 
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