Change brake fluid from DOT 3 to DOT 5

[stvaughn]

What is the procedure for flushing out the brake system when changing from DOT 3 to 5?

Thanks, Steve

[JFranklin]

Use denatured alcohol  after blowing out all the DOT 3. I use a large syringe and flush each line until they are free off all the old. Drain and flush the master cylinder. Blow out everything to dry. Add the new and slowly bleed each line and make sure the new is clear from each cylinder. 

[c49er]

Flush all steel brake lines 100% with denatured alcohol after removing all rubber parts in calipers/wheel cylinders and MCyl.

Replace all rubber parts and hoses, refill with DOT 5 ... bleed system carefully and slowly.

Done right.

[Str8-8-Dave]

I understand very much why you would want to change from Dot 3 ester alcohol to Dot 5 Silicone fluid, it wont rust, right?  Well from what I have read that's a definite maybe.  We all know the alcohol in Dot 3 does absorb moisture over time but the alcohol readily mixes with both the water and the ester oil so there is no concentration of water and if you replace it ever 3yrs or so it won't absorb enough to rust.  Silicone is inaqueous, won't mix with water.    If water gets into the system  from any source it remains as concentrated water because the silicone cant absorb or mix with it.  A lot of race teams refuse to use it because if that water gets gathers in or near brake cylinders or calipers it can boil on hard braking and create steam which then causes brake failure at the time they are needed the most.  I used silicone fluid in trailers equipped with surge brakes and a couple of antique motorcycles because they sat and never really had a problem but it is an interesting and controversial theory.  The OEM car manufacturers still use ester alcohol in new cars with anti-lock brakes and tell you to flush and replace it every few years to protect expensive anti-lock pumps.  Wonder why they don't got to silicone?

[joe_padavano]

I've used DOT  in an older car previously, and had no issues over a 5-6 year period of ownership. Given the uncertainties that have been brought up, I now use DOT 4 in all my cars.

[PFitz]

1 hour ago, Str8-8-Dave said:

I understand very much why you would want to change from Dot 3 ester alcohol to Dot 5 Silicone fluid, it wont rust, right?  Well from what I have read that's a definite maybe.  We all know the alcohol in Dot 3 does absorb moisture over time but the alcohol readily mixes with both the water and the ester oil so there is no concentration of water and if you replace it ever 3yrs or so it won't absorb enough to rust.  Silicone is inaqueous, won't mix with water.    If water gets into the system  from any source it remains as concentrated water because the silicone cant absorb or mix with it.  A lot of race teams refuse to use it because if that water gets gathers in or near brake cylinders or calipers it can boil on hard braking and create steam which then causes brake failure at the time they are needed the most.  I used silicone fluid in trailers equipped with surge brakes and a couple of antique motorcycles because they sat and never really had a problem but it is an interesting and controversial theory.  The OEM car manufacturers still use ester alcohol in new cars with anti-lock brakes and tell you to flush and replace it every few years to protect expensive anti-lock pumps.  Wonder why they don't got to silicone?

 

Not just a theory. Moisture gets into many old brake systems because the reservoirs do not have a rubber sealed cap like modern cars. As the brake pedal moves fluid in and out of the master cylinder and reservoir, outside air is pumped in/out also.  I've had to  open and inspect the DOT5 filled systems of several customers late 20s and early 30s cars over the years and every one of them showed signs of water pooling and rust in the lowest parts of the system - such as the bottom of the master cylinder and the wheel cylinders.

 

And that's not the only way water gets in.

 

I had a nice chat with a Wagner Lockheed engineer about 25 years ago about brake fluids. He said that Wagner did tests with brake hoses and found they can't seal out water molecules, which are some of the smallest in nature. The molecule's can penetrate right though the hose walls because of vapor pressure being higher on the outside in humid weather. With DOT 5 it can't deal with that water vapor so the water collects in the lowest points. That can lead to boiling in heavy braking because unlike DOT 3 & 4 the water isn't mixed with the DOT5 so it's boiling point is very low. If it boils into steam it greatly expands.  And it not only causes rust, that water can freeze in winter. Any  frozen water being pushed around in cylinders might cause brake system problems.

 

If your going to use DOT5 in your car don't assume it's good for life. Thinking that you never have to inspect inside the brake system, or flush with new DOT 5 every few years, may shorten your life.  

 

Paul

Edited November 16, 2018 by PFitz (see edit history)

[Bloo]

The reason you don't see DOT5 much in racing is that it is such a pain in the ass to bleed. Nobody has time for that trackside.

 

1 hour ago, PFitz said:

With DOT 5 it can't deal with that water vapor so the water collects in the lowest points. That can lead to boiling in heavy braking because unlike DOT 3 & 4 the water isn't mixed with the DOT5 so it's boiling point is very low. If it boils into steam it greatly expands.

 

But that is almost exactly what happens with DOT3. The damage, and the worst fluid, is always at the bottom of the cylinder. DOT5 does not chemically attract water like DOT3 does, and in old open-to-atmosphere systems the water problem is far less severe than it is with DOT3 because far less gets in. If you take apart a DOT5 system often (not always) there is no rust damage at all.  The US Government has been using it for decades for exactly that reason.

 

1 hour ago, PFitz said:

 

If your going to use DOT5 in your car don't assume it's good for life. Thinking that you never have to inspect inside the brake system, or flush with new DOT 5 every few years, may shorten your life.  

 

That is very good advice!

 

 

 

[oldford]

I'm not sure if the problem I had with DOT5 was related to the issue of water, but I had a 1964 1/2 Mustang and replaced the DOT3 with DOT5. Before the change, which included a flush as described above, I never had a problem. After the change, I never got more than 12 months out of a brake light switch. This one is the kind that operated from pressure at the master cylinder. The problem was always related to fluid getting under the switch diaphragm and the switch failed. After the second or third switch failure, I started carrying a spare in the glove box.

 

Frank

[Frank DuVal]

DOT 5 is not used in ABS because of lubricity reasons. 

 

Racers do not like DOT 5 because they "brakes feel spongy". I can't argue with this, as a racer needs the best brakes possible!.And moisture is not an issue for them, since it will get torn apart soon anyway.....?

 

I've been using DOT 5 in collector cars for over 35 years. No issues. No internal rust issues. No driving issues. This includes the collector cars that were used as daily drivers for years, both by myself and customers. 

 

DOT 3 gave me white corrosion, stuck pistons, pitted bores, who wants that crap in their system?  I do not think I ever opened a DOT 3 /4 system for repairs that did not have a problem related to water intrusion. DOT 5 systems do not give me this.  And this "flush DOT 3/4 every 2 or 3 years" is a recent recommendation. 30 years ago no one ever heard of it, since cars were traded every few years, and the fluid was flushed when the cars failed inspection or with brake line failures in salt states....

[PFitz]

3 hours ago, Bloo said:

The reason you don't see DOT5 much in racing is that it is such a pain in the ass to bleed. Nobody has time for that trackside.

 

 

But that is almost exactly what happens with DOT3. The damage, and the worst fluid, is always at the bottom of the cylinder. DOT5 does not chemically attract water like DOT3 does, and in old open-to-atmosphere systems the water problem is far less severe than it is with DOT3 because far less gets in. If you take apart a DOT5 system often (not always) there is no rust damage at all.  The US Government has been using it for decades for exactly that reason.

 

 

That is very good advice!

 

 

 

 

Only after what the Wagner engineer call, "gross contamination". Dot 3 is designed to work with water and protect the system. However, it has it's limits and rarely does anyone stay within those limits by flushing a new quart of DOT 3 through every two-three years in humid climates, as the manufactures recommend, or 5-6 years in dryer climates.

 

Back 25 years ago, that Wagner engineer told me to watch for the new DOT 4 fluids coming from Europe because  they have a much better anticorrosion package. Now good quality DOT 3 has that too. 

 

Yes, the  DOT 5 sellers have used that "the government uses it" line since they first started selling it. But what this Wagner engineer pointed out was that the DOT 5 sellers haven't been telling people is that the government does not leave DOT 5 and forget about it. There is supposed to be a change and flush schedule.  My former brake parts supplier in Farmingdale  Long island  told me he was putting his kids through college thanks to his contract with the US Postal System to repair their truck  brake systems, because they were using DOT5.  

 

Paul

[Bhigdog]

DOT 3 collects water and fosters corrosion and sludge.

DOT 5 does not.

DOT 3 is a very effective paint stripper.

DOT 5 is not

DOT 3 does not hold small bubbles makes bleeding easy

DOT 5 does so bleeding takes more effort

DOT 5 has been in my 69 Corvette since 1990 with no change/leakage/corrosion.

DOT 3 originally  in since it was new caused leaking, corrosion, severely pitted calipers.

 

It's your choice..............Bob

[Bloo]

32 minutes ago, PFitz said:

But what this Wagner engineer pointed out was that the DOT 5 sellers haven't been telling people is that the government does not leave DOT 5 and forget about it.

 

Yeah thats true. They change it.

 

I no longer own anything with DOT5 in it, though I have owned several. The corrosion problems just disappear. No more shot bores, stuck pistons, etc. I wouldn't be terribly uncomfortable at 10 years on DOT5 based on systems I have opened up, but it is highly unlikely I would ever leave it in that long.

 

DOT3/DOT4 on the other hand becomes a maintenance nightmare if you have a bunch of cars because it needs to be changed so often. 5 or 6 years? Not at my house. Once every 2 years is about the limit if you don't want a godawful mess in your cylinders, and I live in a dry climate. Once a year is better.

 

 

 

[PFitz]

3 hours ago, oldford said:

I'm not sure if the problem I had with DOT5 was related to the issue of water, but I had a 1964 1/2 Mustang and replaced the DOT3 with DOT5. Before the change, which included a flush as described above, I never had a problem. After the change, I never got more than 12 months out of a brake light switch. This one is the kind that operated from pressure at the master cylinder. The problem was always related to fluid getting under the switch diaphragm and the switch failed. After the second or third switch failure, I started carrying a spare in the glove box.

 

Frank

 In another post I uploaded the paperwork that came with a new NAPA brake pressure switch that I recently had to install in a customer's car. It said to not use DOT 5 with the switch. I'll see if I can find it and repost it. 

 

Depending on  point of view, and brake system rebuilding expertise, one of the good and bad things about DOT 5 is that, …. if there's a chance of a leak, DOT 5 will find it first. 

 

Paul

[Bloo]

The hydraulic switch problem has been widely reported.

 

Interestingly, Harley Davidson uses switches like that and have been using DOT5 for years. A switch version that will work with DOT5 must exist.

[PFitz]

7 minutes ago, Bloo said:

 

Yeah thats true. They change it.

 

I no longer own anything with DOT5 in it, though I have owned several. The corrosion problems just disappear. No more shot bores, stuck pistons, etc. I wouldn't be terribly uncomfortable at 10 years on DOT5 based on systems I have opened up, but it is highly unlikely I would ever leave it in that long.

 

DOT3/DOT4 on the other hand becomes a maintenance nightmare if you have a bunch of cars because it needs to be changed so often. 5 or 6 years? Not at my house. Once every 2 years is about the limit if you don't want a godawful mess in your cylinders, and I live in a dry climate. Once a year is better.

 

 

 

 For whatever is causing your need  for as little as one year changes, you might want to try a name brand DOT 4. It might have better anti-corrosion additives. 

 

All my brake work customers are within 100 miles of the coast - from New England down to NC.  And they are all drivers, no trailer queens. None have such short term brake system  water contamination problems using name brand DOT 3. 

 

Paul

[PFitz]

12 minutes ago, Bloo said:

The hydraulic switch problem has been widely reported.

 

Interestingly, Harley Davidson uses switches like that and have been using DOT5 for years. A switch version that will work with DOT5 must exist.

I don't know what HD uses, but the switch that says not to use DOT 5 is a NAPA Echlin #113. It's what fits most of the early Wagner Lockheed systems.  

 

Paul 

[Bloo]

I have Castrol DOT4 in two of my cars. I used Castrol GT/LMA (also DOT4) until they stopped making it. The LMA stood for "Low Moisture Activity". Everyone took that with a grain of salt because DOT4 is a different chemical, and attracts water even worse than DOT3 does. Like DOT3, it starts absorbing moisture and degrading as soon as you break the seal on the can (if not sooner).

 

The reason to use DOT4 is that it starts out with a higher boiling point than DOT3.

Edited November 17, 2018 by Bloo (see edit history)

[PFitz]

Ok, found the post about DOT5 should not be used in some brake pressure switches.

It's here,.. the 22 post down.  

 

Edited November 17, 2018 by PFitz (see edit history)

[stvaughn]

And post #23 gives the correct NAPA brake light switch number for DOT 5 fluid SL 142

[Spinneyhill]

8 hours ago, Bloo said:

Everyone took that with a grain of salt because DOT4 is a different chemical, and attracts water even worse than DOT3 does.

Um, they are both based on polyglycol-ether with borate ester additives to raise the boiling point in DoT 4 and DoT 5.1. The borate ester additives would have to absorb water... do they?

 

They also contain corrosion inhibitors to slow corrosion with quantities varying by brand.

[Bloo]

58 minutes ago, Spinneyhill said:

The borate ester additives would have to absorb water... do they?

 

Good question. I don't know. The author of this patent seems to think so.

 

https://patents.google.com/patent/US20060264337A1/en

 

Commercial DOT 4 brake fluids always contain boric acid esters, for example methyltriglycol borate, which can chemically eliminate certain amounts of penetrating water from the brake fluid by hydrolysis. A disadvantage is that boric acid esters are themselves hygroscopic, with the result that such DOT 4 brake fluids, particularly in regions of high relative humidity, for example in tropical and subtropical regions, can very rapidly absorb so much moisture that, in spite of the trapping function of the boric acid esters, the operability of a brake system filled therewith can be adversely affected.

 

[Spinneyhill]

The obvious next question is, who makes brake fluid to that patent?

[c49er]

Use a mechanical switch with DOT 5 if the need be.

[Curti]

I personally prefer DOT5 in all of my Auburns. I did experience brake light switch failure as was described above.  Went to NAPA and found a modern switch which is almost identical to the originals.

That is to say they pass judging at the ACD events.  I have not had a brake light switch failure in seven years.  I would NEVER go back to DOT3

[auburnseeker]

Even if you used original brake light switches,  available all over ebay for 10.00 or less and replaced them every few years,  they are alot cheaper than a brake job using regular dot3 replacing all the cylinders and the affects of the corrosive brake fluid on finishes. I've switched to Dot 5 in every car I do,  regardless of the upfront cost.