If you dig down a little bit what you'll find is there are minimum wet & dry boiling points that the product must meet in order to be sold as DOT 3 or DOT 4 etc. A higher quality product though, will exceed these minimums by a healthy margin. In fact some DOT 3 will exceed the the DOT 4 standard. We're talking 5 bucks for a quart here, buy the "good stuff" and change it periodically.
I just finished reading an article in the June issue of Car Craft. In the section 'Ask Anything' there is a extensive article on brake pads, rotors, and brake fluid. It states"As for fluid, DOT 4 has a higher boiling point than DOT3, but DOT3 will hold up better if you don't do the annual maintenance."
The reason they suggest that is because DOT 4 absorbs moisture ("hygroscopic") a little more readily than DOT 3, therefore, over an extended period of time it can actually end up with a lower wet boiling point. DOT 3 seems to be a little bit thinner in viscosity, seems to bleed easier and has good cold weather performance. (Just because something says "Racing" on the bottle doesn't necessarily mean you want it in your car.)
I run DOT 3 in my cars, my bikes require DOT 4 and I ran DOT 5 synthetic in a previous hot rod and was never happy with the DOT 5. Pedal never felt right with the DOT 5, only thing I liked was that it didn't eat paint.
Dot three and four are both hydroscopic, dot four has a higher boiling point but both are compatible with each other as well as every brand must be compatible with any other Sent from my iPhone using H.A.M.B.
I've always run Dot 3 but my ot daily specifies Dot 4 and it has no issue with the brakes and no issues after some hard driving on interesting and twisty two lane roads. Hunting brake fluid for it I did find that there is a dual compatibility Dot 3/dot 4 fluid from one maker. In fact that was all one parts house had as far as Dot 4 goes and I passed for a brand I trust at another store.
The Federal minimum standards required meeting a certain boiling point temperature for each specification, though today the best DOT 3 will exceed even the minimum standard for DOT 4. I have always had easy trouble-free bleeding with DOT 3 and good hard pedal so it's my go to stuff. One characteristic (maybe you know this) of glycol type Brake fluid is the ability to absorb and disperse moisture. If it did not the accumulation of water (in older open systems) would lead to freezing brake lines in cold weather and boiling steam in high temps, either of which means no brakes at all. It does lower the boiling point however, and is one of the reasons why it should be flushed every few years. Noticed a local dealer shop advertising card mailer had a $70 "special" for what they termed a brake fluid exchange. Guarantee what that is, they take a syringe and suck out maybe 1/2 cup of brake fluid out of the master cylinder and top it off. Wankers
My OT daily sez "DOT 3 Only" right on the reservoir cap. I think the reason for that has to do with the innards of anti-lock type brakes, which have some tiny orifices or somesuch, 4 is too thick or somesuch.
My HD has antilock brakes and calls for DOT 4 so I don't think that is right. They did a recall on some of the early antilock brake bikes and changed the fluid but my 13 wasn't included. They also recommend you change the fluid every two years, I did change the fluid on both bikes as a safety precaution.
Avoid annual maintenance? Must be some good shit lined up on the desks at Petersen Publishing. No wonder I let my mag subscriptions lapse. When you open any brake system up you risk contaminating the system with moisture. Kind of the reason you bleed the system. To remove air bubbles, contaminants and replenish the brake fluid properties. It's not just water you have to worry about. I would rather do regular maintenance, have fresh fluid and find a leak/issue after some expected work than randomly have a brake failure while on the highway. DOT3 is more 'stable' because it already has a lower boiling point. Where-as DOT4 can have a larger drop in boiling point performance over time, DOT4 will still have a higher boiling point. That's why it is labeled DOT4. You leave any glycol brake fluid alone long enough it will become junk. How many times have you opened a master cylinder on a 'barn find' to see nothing but chocolate pudding in the reservoir? If you don't do a full bleed, at minimum remove the stuff in the reservoir and bleed the fluid out of the caliper/wheel cylinders. Anyone who has done heavy braking or an open track day knows, getting brake fluid too hot(boiling) will render it useless after, requiring a bleed just to get the car to stop again.
Not necessarily, remember DOT 4 absorbs moisture out of the air a lot more readily. That's why they wrote the article the way they did. A good quality DOT 3 will exceed even the minimum DOT 4 federal specification. For example, Prestone DOT 3 has a listed dry boiling point of 460° F exceeding the minimum DOT 3 spec by about 50° and the DOT 4 spec by about 15° I don't think this matters too much, the hardcore change the brake fluid before every race or near enough.
The Jalopy Journal
