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DOT 3 Brake Fluid vs DOT 5 Brake Fluid


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So the Master Cylinder has been rebuilt on my '69 Chevrolet, replaced the rear brake cylinders as well. I ordered a complete brake line system for the car, I'm at the point now trying to decide which type of fluid to use. I have two friends that have been restoring / repairing these classic cars for years and each has a different preference on fluid type. I'm only familiar with DOT3...but since it's a new system, I want to look at both types.

 

Any suggestions are welcome.

 

Steve

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Steve, this is not to condemn or recommend, but just an honest report of my experience with Dot 5.  I had a 66 Sunbeam Tiger that we completely re-did from top to bottom, and a 63 Austin Mini ( still have it ) that I adapted a new complete disc brake setup to. Both cars had all new brake parts, were bled and broke in correctly, and both cars VERY quickly had to have the little screw-in brake light switches replaced twice before I pumped all the Dot 5 out and replaced with Dot 3. The brakes on both cars were "kinda" mushy or "soft" feeling with the 5, but never failed. This may be because it was several years ago and Dot5 was "the new stuff", or that I did something wrong, but I have done more cars since, with our 55 Studebaker being the latest recipient of a new disc brake "kit", and no problems ever with the Dot 3.  Remember, this is just my experience, not a slam, as I have friends who use the 5 and are satisfied. Good luck sir !

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Posted (edited)

I will just provide an opinion. I have dot 5 in the 37 and it seems fine, but the car has been undergoing restoration for years and has not seen much road brake use. The biggest thing to consider between dot 3 and dot 5 is the water absorbing with dot 3. If the car is going to be used often Dot 3 is going to be fine. One thing I found with dot 5 was the bubbles. Dot 5 is easy to cause bubbles in by pouring to fast or roughly. When you pour dot 5 watch for the small bubbles in the reservoir. If they are there wait hours for them to dissipate. THIS IS A MUST!!! If you do not you will end up with the air in the system and spongy brakes.  I have not had any problems with brake switches. Your results may be different.....  Just my thoughts, but the water issue will always lead me to dot 5.

Edited by 37_Roadmaster_C
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My preference is to use Dot 5 if the car is not going to be used regularly.

I always use Dot 3 in my daily driver.

Dot 5 will affect pressure brake switches.

I've found no difference in the driving of a car with either 3 or 5. To me the brakes feel the same.

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I use DOT 4 in all my vehicles because it is easy to get at NAPA, but DOT 5.1 sounds like the better choice. Unlike DOT 5, 5.1 is backward compatible with DOT 3 and DOT 4 and has none of the undesirable attributes.

 

 

Screenshot_2021-05-21 What's the difference between DOT 4 and DOT 5 1 brake fluid (1).png

Screenshot_2021-05-21 What's the difference between DOT 4 and DOT 5 1 brake fluid .png

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Dot 5 once and forget about it.  Dot 3 and replace everything again in a few years if you live in a Humid area.  Been using Dot 5 for 25 years and have never redone anything I used it on. Now I can tell you how many cars have come in my possession that the first thing they needed (even with 3 year old brake jobs) was a complete overhaul.  Seized cylinders.  Rust pudding in the master cylinder,  not to mention if you get some on stuff it won't eat the paint up or rust the cast components.  I paint all my parts cast gray use silicone and they still look new 10 years down the road.  Use 3 and it rust the raw steel or peels the paint you put on anything off.  

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Restored a '35 Auburn in the early 1990s with new everything. Used Dot 5 and I've never had to add any, despite a lot of use and sometimes long periods between use.

 

Big benefit to me is that Dot 5 does not eat paint. It always seems like when bleeding brakes that you'll have a little fluid get on backing plates and axles. No problem with Dot 5.

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1969 corvette using dot 3. By 1985 all 4 calipers rusted and leaking. Replaced calipers and entire system using dot 5. Now 2021. Same dot 5, same system, zero maintaince, zero problems.

1939 chevy pick up. New everything in 1999 using dot 5. Now 2021. Same dot 5, same everything, including brake switch, zero maintaince, zero problems.

You do the math......bob

 

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  • 2 weeks later...

I have three cars now with Dot 5. A 57 Tbird with no problems except the brake light switch in 25 years. Probably 10 years in a 70 Vette and 3 years with a 66 Impala. The key is to start with all new components.

 

Tom

 

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On 7/1/2021 at 8:36 AM, joe_padavano said:

Unlike DOT 5, 5.1 is backward compatible with DOT 3 and DOT 4 and has none of the undesirable attributes

And 5.1 does NOT have the one attribute I'm looking for in a brake fluid. No white corrosion on internal brake parts! DOT 5 systems are so clean internally. One must start with a clean system, but then it stays clean, unlike all those DOT 3,4, 5.1 glycol based systems.

 

I want to meet the collector who flushes their DOT 3,4,5.1 systems every two years.😉

 

On 7/1/2021 at 12:53 AM, John Byrd said:

This may be because it was several years ago and Dot5 was "the new stuff",

 

I've been filling clean (new or rebuilt) brake systems with DOT 5 since about 1980. No issues. But I don't have any systems with pressure switches for the brake lights.

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Used Dot-5 when completely rebuilding the brake systems on our 1958 and 1963 Chevys, and never had a problem with either one for many, many years, owning and driving tens of thousands of miles.

 

If not doing a complete rebuild, I always use only Dot-4 since it is less hygroscopic (not absorb moisture to cause rust in system) than Dot-3, and is available everywhere - even Wal-mart, since we drive our cars cross-country.

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