The Stovebolt Site Forums The Shop Area Engine Shop DOT 3 vs. DOT 5

Just read the article on brake fluid in the latest "Vintage Trucks". I was leaning to DOT 5, but after reading the article it's definitely DOT3 for me. Basically they were talking about rubber problems w/ DOT 5 and lubrication. Dot 3 expands the seals slightly and creates a tighter system. 5 does not and can cause leaking if the entire system has not been outfitted with the lastest compounded seals and hoses.
Good Magazine. My only complaint is that there seems to be as many cars as trucks!

I read the same article. I am not a rocket scientist so I did not understand half of it. I do know that I am using DOT 5 without any trouble at all. No spongy pedal, no leaks, no paint bubbled off my frame if I spill a little..

DOT 5 is not susposed to asorb moisture like dot 3&4 will. It also has a higher boiling point...less brake fade.
Hard to find too....

What I took away was that if there were any rubber parts that may be NOS or original to the truck, There couls be a leaking problem due to the DOT5 fluid not expanding older type rubber evenly or worse, reacting to older type rubber therefore causing leakage.

I know alot of makers of DOT 5 recommend changing the flex hoses and flushing the steel lines before using DOT 5. I,ve heard that the post office and military have used it since back in the seventies. I have heard that it won't work in these new anti-lock brake systems and it will sometimes mess up brake light switches that screw in master cylinder.

Never have had a problem in the past,but have been hearing a lot of bad stories. So since I just converted from 4 wh drum to 4 wh disc I'm going to dot 4. A couple of reasons are, 1- the aftermarket brake people will not stand behind any warranty if silicone fluid was used, 2- high altitude has an adverse effect on silicone and I'm going to tour through the rockies this summer. I sure don't want to experience brake failure coming down a mountian.

Whitelighting, I dont know when the military started using dot5 . But I do know when I joined in 1989 everything hydraulic had DOT 5.
Brad

DOT-5 is silicone based. I've always been under the impression one can not mix dot-5 in a dot 3 or 4 system.

Like you all said, the seals and stuff will leak. Dot 3 and Dot 4 are compatable.

My opinion, stick to 3 or 4 and just flush the system every few years. It really isn't that difficult.

Jay

Yeah you don't want to mix DOT 5 with DOT 3 or 4. Thats one reason they recommend changing flex hoses, so you don't contaminate the DOT 5 when converting.

Never had good luck with dot 5 silicone based. The only thing good about it was that it did not eat paint. Used it in the race car one time NG(about killed myself half way through a 100 lapper).I have found the dot 4 to be the best with dot 3 not that far behind.The one thing about brake fluid.(i know i said this before) is buy it in small cans(not at swap meets either) and throw what you dont use away.I absorbs mosture real quick and your dot 4 will be dot 3 in no time. So....the BS about changin your brake fluid every couple of years aint BS its a good idea. Castrol GTX brand seemed to be the best so far. If you have ever used dot 5 and than gone back to 4 you will also notice brake pedal pressure being diff.I know all the vette guys and engineer guys love the stuff but in real world driven (your old truck or race car) DOT4 is the one for me.

Also if you have a power brake booster and it leaks a bit it can draw the stuff into the motor.Dot's 3&4 burn harmlessly but Dot 5 results in sand buildup in your motor. Not a good thing

Silicone is not the same thing as silicon (sand) at all.

Actually, sand is silica, an oxide of silicon. One of the combustion products of burning silicone (a polymer of silicon) is silica. Try it sometime, squeeze out some silicone caulk and burn it with a propane torch.

Tagging onto Phat's post...

When I redid the brake system in my '41 De Soto about 4 years ago, I used DOT-5, enticed by the promise of "no moisture absorbtion", and had a whale of a time bleeding the system out; no one (especially the manufacturer) warned me about "not aerating the fluid"!
That means: "pour carefully while filling the MC; don't create bubbles;if using a pressure bleeder, make sure is has a barrier diaphragm etc.".
Well my pressure bleeder didn't have a diaphragm, and I unwittingly made a DOT-5 frappe! :mad:

I finally let the car stand for about two days then went back and pedal-bled and it seemed to be ok.

Am also on my third brake-light switch, BTW.

The news about rubber "incompatability" further concerns me- I think that I will go back to DOT 4 & 3 fluids in the future when doing total system overhauls, and flush the system every couple years thereafter.

As far as paint damage goes, there's nothing wrong with using a funnel to help get the fluid into the MC only .

What's the difference in pedal pressure between Dot 4 and Dot 5?

Does Dot 5 provide a firmer pedal? Is that why Vette guys like the stuff?

Jay

no the pedal is softer with dot 5. Phat mentioned the Castrol according to an article I read ***** super duty dot 4 is supposed to be the same thing.

I have herd about a high performance Dot 3 brake fluid too.
I saw it in a Corvette brake product catalog. It is
"Motorcraft Xtreme-Duty DOT 3 BLUE brake fluid". It has a Dry boiling point of 550' and a wet boiling point of 284'
Have any of you used this stuff before??

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Found this webpage on brakes. Even though it was written for ferds, it has some good info. Brake Fluid 101

What is the difference between Dot3 and Dot4 fluid? Can Dot4 be used without flushing system?

Mark

Dot4 is Dot3 with a higher dry boiling point. It is a bit more hydroscopic so use Dot3 unless you are racing or your vahicle specifically specifies it. You can mix them if you want.

I had nothing but trouble with dot 5 in my '38. Brand new Wagner wheel cylinders gave up the ghost in 5 years (taking out my brake shoes with it of course) and stoplight swithces didn't last very long. Not to mention the extra cost of replacing the fluid everytime I repaired a leaking component. I went back to dot 3 and will try to remember to change it every other year.