I may be opening a can of worms here, but I am getting closer to installing brake lines, etc on my 56. I restored a 71 Cutlass years ago and decided to go with the DOT 5 brake fluid in it. I also put a whole new brake system on a 65 Coronet and put in DOT 5 fluid. I asked about doing this years ago on the Classic Oldsmobile site and got all kinds of answers. The advantage to me in using the DOT 5 vs. DOT 3 or 4 is it won't mix with water, as in it's not hydroscopic. It doesn't draw moisture out of the air. I'm sure you have noticed if you bleed the brakes of a older car the old fluid squirting out of the bleeder is brown, not clear as the new fluid is. That of course is rust that has came off of the steel parts in the system because of the h20. Also it is standard practice to not keep a opened bottle of brake fluid for years and years because the fear is water will get into it from the atmosphere. Also if you spill regular brake fluid on floor it will rinse right off with water.
DOT 5 is also called silicone brake fluid and is not hydroscopic, it won't mix w/ water. But apparently there is more to the story, something about different boiling temperatures/ properties, etc. Anyway I did use it in both the 71 and 65 and no troubles. Of course neither car has big race type brakes on them and my rotors never got red hot like you see on the Nascar racers....
I can't say that there is any problem with rusty brake fluid but it just seems it would be better to not have. Most cars only last 10-15 years and it's no problem.
DOT 5 is harder to get you won't find it at Walmart, but most auto parts stores can get it. It is more expensive. Also if you change over you have to make sure all of the old " regular" brake fluid is flushed out because they won't mix. Which is why when making a entirely new system that's the time to do it.
Any thoughts ?? Thanks Greg

 

I used DOT5 on a couple T birds many years back when I did brake restoration on them. DOT5 was the new thing at that time.

 

I used Dot 5 when I built my 53 F100. The only downside I have heard of is any moisture that enters the system will collect in the bottom of the wheel cylinders, causing them to corrode. If used regularly this shouldn't be a problem, but many of our old truck see little use. I've read that Dot 5 is slightly compressible, causing soft pedal, but in 9 years and 90K miles I haven't noticed any differences.
Harley Davidson has used Dot 5 for years. When I pulled the brake system apart on my '89 Softail Custom last year, the calipers and master cylinders looked new inside.

 

I used it successfully in my 48 F-3 with a Chrysler MC. Worked great. If you have a car/truck that sits for 5-6 months of the year like here in New Hampshire the brake system will last a lot longer.

 

I was going to use DOT 5 on my truck with the original drum brakes, but since I changed the front brakes to discs, I decided to use DOT 4. I was told right or wrong, that since DOT 5 does not absorb the water that gets into the system, so it will push it to the lowest point which is the wheel cylinders. The water will boil in the cylinders of the disc brake calipers and compromise the braking. I don't know if that is true or not, but I decided not to take a chance.
Mark

 

It's actually spelled Hygroscopic, for whatever reason.

Glycol type fluid was chosen because it absorbs small amounts of moisture, not in spite of it. So it's amusing sort of, to hear that.

If it didn't, would boil at high temperatures and freeze solid in the winter. Just a tiny slug of water is enough to do this. Not good. The old cast iron brake system components were relatively open to the atmosphere and tend to sweat with temperature changes and high humidity. The moisture is going to get in there, the question is what happens after that.

 

Had it my truck for 5 years before I sold it with no problems. New owner has had it since 2012 and the same fluid is still in there. Truck sat from November to May every year when I had it due to harsh weather and the worst thing you can do for any brake system is let it sit...

 

I think its a bit like waterless coolant and Apple products. Great in theory but once you've invested you can't change and they go on fleecing you safe in the knowledge


I'm just kidding. Both systems work well and if you are doing a system from scratch it has its advantages with your new paint. One thing I hate about Dot 4 is the way it will eat its way through most hoses in time if you have a remote reservoir. I know you can get stuff that's meant to be resistant to it but its not much comfort when it starts weeping anyway.


Also stay away from Dot 5.1. Definitely not to be confused.

 

What is "it"? Hardly anybody changes conventional brake fluid, that's not news. Industry standard is 2 to 3 years.

 

That actually makes a lot of sense. I knew there must be a reason and the way you and the others explained it I think I understand. My question was why not have a brake fluid that doesn't absorb water as Dot 3 does. So Dot 5 to the rescue! But wait! What about if water does get in the system and then the drop goes to the lowest spot? Now I see. Thanks for the simple layman answer. Though I think the way our collector vehicles are used and pampered Dot 5 would probably be good. But our old trucks (and cars) back in the day, that were just every day work vehicles with master cylinders mounted below the floor probably were very likely to get water in the system, especially with mud and slush getting splashed around down there.

 

What is DOT 5.1? I don't think I've seen that. Thanks to all who responded. I'm still not sure what to use. I have new calipers, wheel cylinders and lines so all I would have to do is flush the master cylinder if I decide to use DOT5.

 

Aircraft brakes commonly use a type of petroleum base-stock hydraulic fluid. But the entire brake system is completely sealed. I've wondered this myself, why the engineers went the way they did with autos. I like DOT 3 because it bleeds super easy, the viscosity is fairly thin.

DOT 5 is actually kind of a pita, I looked into it when redoing everything, it makes a certain sense for collector trucks I'm sure. I flush the brakes every year or so - even a gravity bleed is super easy and only a few bucks.

 

IT???? "It" is Dot 5 which if you had read the entire thread you would have known what "it" is...

 

Here is an article on brake fluid from Hot Rod that may be of help to us all.

Changing Brake Fluid - Hot Rod Network

As always, pick your poison.

Philip

 

I have been using Dot 5 in vehicles for decades now. And one has had Dot 5 in it continuously for over 30 years with no component or fluid replacement in that time. The braking system still to this day functions as and looks as new internally.

Dot 5 install is best done on a fully new brake system where there are no traces of conventional brake fluid in the system. If it is mixed with conventional glycol / borate ester brake fluids the mixture will turn to snot. Then you got a real mess on your hands.

As for the issue of water settling in to the low spots of a braking system with Dot 5 lets look at this closely.

There is only one place for moisture to enter the brake system,the master cylinder, it is the only component that can be open to the atmosphere.
Most all 2 circuit braking systems are essentially air tight with full gasketing on the master cylinder. This is to minimize the absorption of water in to hygroscopic brake fluids.
Since Dot 5 is not hygroscopic and the brake system should only be filled with brake fluid there is no physical way way for moisture to get in to brake lines and wheel cylinders if the system was filled correctly..

Second Most all cast iron master cylinders do not have the inlet to the piston as the low in point in the reservoir. This is to minimize the intrusion of debris (rust) in to the piston assy. If you happen to get moisture in to a dot 5 system in the master cylinder through condensation (only an issue with metal reservoirs) the water will settle in to the low spots of the master cylinders reservoir and is plainly visible as globules of water sitting in the bottom of the master cylinder in the low spots. This is an easy fix simply by sucking the brake fluid out of the master cylinder and soaking up the water with some paper towel and refilling the master cylinder.

So that issues of dot 5 rusting brake lines and master cylinders in low spots is a myth.

In the one vehicle that has Dot 5 in it continuously since 1985/86 the braking system is sporting ALL its original components the vehicle is not in heated storage and can see temp swings of up to 30° C in a 24 hr period. There has been zero water intrusion in to the system.

I have not ever had an internal moisture related (corrosion) failure in any vehicle I have installed DOT 5 in to.

Dot 5 does have higher compressibility but this would only be noticeable in very high performance braking systems and even then barely or systems that were not filled correctly..

Dot 5 does aerate very easily so care must be taken when filling a braking system. The Master cylinder needs to be bench bled with slow compression's over the span of a couple days to insure that any aerated fluid is purged.

When filling the master cylinder you must allow for any air bubbles to come out of suspension before bleeding the rest of the system.

The system should bled 2 additional times with at least a 24hr rest between bleeding's to insure that there is no air in suspension in the system.

If this regime is followed you will have a braking hydraulic system that is maintenance free with component life measured in decades not years.
DOT 5 is not suitable for use in modern anti lock brake systems due to the lower lubricity of Dot 5.

If the reservoir is sized properly there should be no need to ever open the system up to add fluid, minimizing the potential for moisture intrusion.

I also recommend the addition of on or near the master cylinder a label/sticker stating USE ONLY DOT 5 FLUID Or USE ONLY SILICONE BRAKE FLUID. This will prevent someone from inadvertently putting conventional fluid in the system.

Dot 5 is more expensive but since it does not ever need to be refreshed the additional cost is negated with the first scheduled flush that conventional brake fluid would need.

Any brake components make in North America after 1972 will be Dot 5 compatible. Earlier NOS or NORS brake components may NOT be Dot 5 compatible and the seals could swell and soften when exposed to Dot 5 brake fluid.


Dot 5 was developed for the US military to combat corrosion issues in the braking systems of equipment as many pieces of equipment see infrequent use and could sit idle for months or even years.
The issue they were having was equipment would site idle or be stored for extended periods of time and then when called on for use the braking systems would have corroded internally and the vehicle would need brake components replaced before being put back in to service.
The implementation of DOT 5 negated this issue.

Dot 5 was designed specifically for equipment that was idled or stored for long periods. So for many of our trucks it is the perfect fit. Yes it is a PIA to convert to and fill brake systems with Dot 5 but the pay off is a brake system that will essentially be maintenance free and will extend the life of components from years to decades.