dot 5 brake fluid

[Str8-8-Dave]

Dot 3 is alcohol/castor oil based and the alcohol DOES absorb moisture.

 

Dot 5 is silicone which WILL NOT absorb moisture.

 

The Dot 3 stuff can and does absorb quite a bit of moisture but since water and alcohol bind perfectly the moisture content is spread over the entire volume of the brake fluid and is much less likely to boil causing brake failure during a panic stop.

 

With silicone, any moisture that does collect in the system forms droplets which are not absorbed and migrates to the lowest point in the system, I.E. the wheel cylinders.  This can result in boiling causing a compressible gas known as steam to form from the heat transmitted to the fluid in the wheel cylinder, particularly with disk brakes.  This can be an awkward occurrence during a panic stop.  This, combined with owner reluctance to replace brake fluid every 1-2yrs as recommended by the OEMs is why the OEMs do not use silicone.  They are not fond of lawsuits.

 

My late model Porsche has extremely sophisticated brakes and anti-lock systems that can be severely damaged by moisture absorbed by the Dot 3 brake fluid installed at the factory.  The Dot 3 brake fluid is replaced in annual factory authorized service encounters to avoid this.  Porsche specifically DOES NOT recommend Dot 5 and won't warrant the brakes on a car using it.  Also, if you want to participate in Porsche Club of America club racing, you will be asked to produce a receipt showing your brake fluid is no older than 1yr in service.  Failing that you have 2 options, don't participate or let club mechanics change the brake fluid on-site.   

 

All that said I have used Dot 5 in the past without incident.  I have also purchased Dot 5 in Harley Davidson TM cans.  I have no idea if they still carry it.   

[auburnseeker]

I'm replacing all the new brakes on the 33 Plymouth, with all new brakes,  because they didn't use Dot 5 and everything is rusted up.  What a shame.  Parts look new on the outside and all rusted up on the inside.   I can't believe anybody with an old car that replaces everything,  would even think of putting Dot 3 in.  Last 3 vehicles I bought were all the same.  Throw out all the new parts to put in all new parts again.  

[Terry Bond]

I think we've beat this one to death over the years.

I use DOT 5.  No way was I going to risk paint-eater stuff on a newly restored, freshly painted car.  We honed and redid wheel cylinders ourselves, flushed and dried all lines, put in la new MC and added the fluid.  Took a couple of tries to bleed the system (as is common with DOT 5) but we've done well since with NO issues.  Oh-forgot to mention the car is a 74 MGB GT finished in 2006 with a shake-down run from Va Beach to Gatlinburg TN through the mountains.  50K miles later, were still enjoying our DOT 5 with no issues.  Just finished over 860 miles with it driving through the mountains to Beckley WV for the AACA Meet there and back.  Mountain roads all the way.  Never missed a beat.  We've never trailered it to a show.  Routine maintenance and regular inspection (along with copious amounts of cleaning and polishing) keep it lookng and running great.

My 10 cents worth on the issue (again).

Terry

Edited July 1, 2022 by Terry Bond (see edit history)

[Bhigdog]

My 69 vette. 25 years ago all calipers rusted and leaking. Replaced the entire system and used dot 5. 25 years later same fluid, no leaks, or repairs, only taking a peek at the level every few years. Never needed to add. 

Whats not to like......bob

[auburnseeker]

Does VW use Dot 5 now?  I went into the parts store and asked for Dot 5,  as it wasn't on the shelf. and the guy said,  oh you must have a VW,  No I said Old car,  I just don't want to do the brakes every 3 years and throw all the parts out.  

Curious how a Dot 5 system would get water in it?   Also I heard the reason for Not using 5 was because of a lower boiling point which could cause brake failure in racing applications.  Which 99.9% of our cars,  even at a daily driver level will never see. 

[John348]

53 minutes ago, Bhigdog said:

My 69 vette. 25 years ago all calipers rusted and leaking. Replaced the entire system and used dot 5. 25 years later same fluid, no leaks, or repairs, only taking a peek at the level every few years. Never needed to add. 

Whats not to like......bob

Same results as Bob, looks just as clean as it did 30 years ago when I rebuilt the entire system, What is not to like?

[Marty Roth]

Agree !

What's not to like?

 

Same results as Bob and John-Great results with Dot-5 on several cars,

Not so much on others with Dot-3

Dot-4 is less hygroscopic (ability to retain water) than 3, and is in some of our cars - better than Dot-3, but surely not as good results as Dot-5.

Caution, DO NOT MIX 5 with either 3 or 4.

Additional note - Dot-5.1 is NOT the same as Dot-5.

Edited July 1, 2022 by Marty Roth (see edit history)

[Larry Schramm]

7 hours ago, Bhigdog said:

My 69 vette. 25 years ago all calipers rusted and leaking. Replaced the entire system and used dot 5. 25 years later same fluid, no leaks, or repairs, only taking a peek at the level every few years. Never needed to add. 

Whats not to like......bob

 

Our Corvette too.  Also has the best pedal it has ever had.

[cudaman]

I ran DOT5 in my 1971 Plymouth GTX for the 20 years I owned it.  Firm pedal and never had an issue. 

[Frank DuVal]

16 hours ago, Flivverking said:

What does that tell you? 

That many people cannot use DOT 5 properly! They have bad bleeding techniques. I have seen it in person, wild slamming of the brake pedal, sure to aerate the fluid and cause spongy pedal, then blame the rebuilder.

 

Never had a problem using it in my and my customers cars for over 40 years.

 

Of course I could use DOT 3 and flush every old car I own every year. I don't have time for that. Do it once, do it right, drive on safe.

[intimeold]

Yes, DOT 5 is the way to go; at this time.

 

But never add DOT 5 to a previously DOT 3 system; that hasn't been cleaned out completely   

[Glen Andrews]

In 1991 I put silicone dot 5 in my 1976 Corvette after replacing everything, brakes worked fine, sold the car in 2008. In 1995 I put silicone fluid in my 1938 Chevrolet pickup, of course after replacing everything, 27 years later and still doing fine. One problem though was with the original type hydraulic brake light switch, it took a lot of pressure on the pedal until it would activate the brake light, so I rigged up a mechanical switch. A couple of months ago here on the Forum there was a thread about the original type hydraulic switch and that problem, and someone mentioned a switch that operated with less pedal pressure. One was a Harley Davidson part and the other one was from Ron Francis Wiring. I got the one from Ron Francis, installed it, and it does activate the brake light a lot quicker than the original type; and, the mechanical one I rigged up is history. There's a lot of great information and knowledge contained on these Forums.

[3macboys]

5 minutes ago, intimeold said:

Yes, DOT 5 is the way to go; at this time.

 

But never add DOT 5 to a previously DOT 3 system; that hasn't been cleaned out completely   

How do you properly clean out the system?  The reason I'm asking is that someone (not me) decided to fill the system on my 30 Dodge with DOT 3 or 4 since DOT5 wasn't available off the shelf right at that moment, and off course not everything was tightened up at the time so leaks abounded along with peeling paint.  The wheel cylinders and master cylinder were all freshly rebuilt and all new lines ran before the initial filling.   

[intimeold]

1 minute ago, 3macboys said:

How do you properly clean out the system?  The reason I'm asking is that someone (not me) decided to fill the system on my 30 Dodge with DOT 3 or 4 since DOT5 wasn't available off the shelf right at that moment, and off course not everything was tightened up at the time so leaks abounded along with peeling paint.  The wheel cylinders and master cylinder were all freshly rebuilt and all new lines ran before the initial filling.   

Well. 

 

Certainly in your case the wheel cylinders would have to be gone through again.  Not a really big job.

 

The master cylinder: Same thing, just do a fresh rebuild.

 

I suppose the lines could be flushed.  We have had good luck, doing that.

 

None of that is really a big intricate job. Just time consuming.

 

Then after the rebuild, maybe a 1000 miles or 1 year or so later; just flush the system by bleeding fluid out at the bleeders. Don't let the master cylinder go dry.

 

intimeold

[Glen Andrews]

Basically you start all over with all new parts, any part that has rubber will need to be replaced if it has been exposed to DOT 3 or 4 brake fluid. It would be nice to also replace the steel lines, but if that is not possible then the lines need to be cleaned out, years ago on my '76 Corvette I used denatured alcohol and ran that through the lines a couple of times, blew them out, then put in some silicone DOT 5 in the lines and blew that out, let them sit overnight (in the garage). The wheel cylinders and M/Cyl don't have to be replaced but the rubber seals and such, also brake hoses, need to be replaced.

[3macboys]

11 minutes ago, intimeold said:

Well. 

 

Certainly in your case the wheel cylinders would have to be gone through again.  Not a really big job.

 

The master cylinder: Same thing, just do a fresh rebuild.

 

I suppose the lines could be flushed.  We have had good luck, doing that.

 

None of that is really a big intricate job. Just time consuming.

 

Then after the rebuild, maybe a 1000 miles or 1 year or so later; just flush the system by bleeding fluid out at the bleeders. Don't let the master cylinder go dry.

 

intimeold

 

5 minutes ago, Glen Andrews said:

Basically you start all over with all new parts, any part that has rubber will need to be replaced if it has been exposed to DOT 3 or 4 brake fluid. It would be nice to also replace the steel lines, but if that is not possible then the lines need to be cleaned out, years ago on my '76 Corvette I used denatured alcohol and ran that through the lines a couple of times, blew them out, then put in some silicone DOT 5 in the lines and blew that out, let them sit overnight (in the garage). The wheel cylinders and M/Cyl don't have to be replaced but the rubber seals and such, also brake hoses, need to be replaced.

Thanks fellas - sounds like a pay me now or pay me later proposition - right now the body is off the chassis so the work can all be accomplished from above and replacing the lines is likely the most efficient thing time wise, maybe even monetarily.   

[auburnseeker]

Dot 5 would especially be good in one of those long term restoration projects where you work on the ting for 10 years.  You restore the chassis and maybe even get it running and driving,  then because of time and money spend another 5 years doing paint upholstery and final assembly,  before you actually get it on the road.  With Dot 5,  the brakes will still be like the day you did it,  where 3 or 4 you will be doing a brake job on your freshly restored car that you haven't even driven yet. 

[Marty Roth]

On 7/1/2022 at 8:14 PM, Glen Andrews said:

Basically you start all over with all new parts, any part that has rubber will need to be replaced if it has been exposed to DOT 3 or 4 brake fluid. It would be nice to also replace the steel lines, but if that is not possible then the lines need to be cleaned out, years ago on my '76 Corvette I used denatured alcohol and ran that through the lines a couple of times, blew them out, then put in some silicone DOT 5 in the lines and blew that out, let them sit overnight (in the garage). The wheel cylinders and M/Cyl don't have to be replaced but the rubber seals and such, also brake hoses, need to be replaced.

Agree !

Denatured alcohol is the way to go in flushing the lines.

back in ( I believe) the mid-late 1980s, when we did the Dot-5 brake conversion on my 1958 Bel-air, we flushed with denatured alcohol and were warned NOT to use an air compressor to blow out the lines, but to let it evaporate naturally. I REALLY LIKE @Glen Andrews' suggestion of adding Dot-5 and blowing that out. My only reservation is that the compressed air might again introduce a minimal amount of condensate - likely not a serious concern, but just me wearing belt and suspenders !

[Glen Andrews]

Good point about the compressed air, I guess I got lucky when I did my '76 Corvette. When I did my '38 Chevy pickup all the brake components were new so no need for compressed air. There are other good points brought up previously in this thread pertaining to bleeding procedure and the handling of the Dot 5 Silicone. Once fluid is in the system I start with gravity bleeding at the wheels, and then with an assistant to pumping the pedal but not with the force of a gorilla, and of course bench bleeding the master cylinder first. And pour the fluid slowly and if beforehand you drop the container of fluid then set it on the bench and wait a few days to let the bubbles work out and then get back to the brake job. As I said, you and several others in this thread know the characteristics of Silicone Dot 5, and if done correctly the brake job will work trouble free for many years; do it right and be done.

[mrspeedyt]

 I never changed the dot three brake fluid on my 41 Cadillac that I bought in 1985 until last year when I had to rebuild the master cylinder.  yeah...  the old fluid was dark. But until the master cylinder leak everything has worked fine. But I live in northern Arizona.  not a whole Lotta humidity in the air.  and in my daily drivers that I’ve had over the years I’ve never rebuilt anything or had any problem with leaks.

dot 3.   where I live it’s OK.

Edited July 5, 2022 by mrspeedyt (see edit history)