What's up with DOT 4 brake fluid?

[bdcardinal]

Originally Posted By: The Critic

I suspect that it is because most applications requiring DOT4 cannot take an over-the-counter DOT4 fluid. Most of the DOT4 applications require a DOT4-LV fluid.

It is OTC at my work. I make sure we keep it in stock. Now telling owners their car requires DOT4LV is a whole different story.

 

[tommygunn]

Castrol's DOT4 brake fluid is supposed to be very good, if you can find it! Usually around $10/qt I believe the viscosity is fairly low so you can use it in LV cars and has higher boiling points than the DOT4 minimum. DOT 5.1 may be easier to find than DOT4 LV and has even higher boiling points (500 dry, 356 wet). Napa seems to be the best place to obtain DOT4 brake fluid, both LV and non-LV. They have Pentosin, Valvoline, and their own house brand DOT 4, probably the only store that carries three brands of DOT 4! Sadly, Napa doesn't carry Castrol brake fluid. (They also have Napa DOT 5.1, but it's hideously expensive at $25/qt; just get it online or at O'Reilly if you're near one). If you can get to O'R, check out Wagner DOT 5.1

 

[artificialist]

Having fluid labeled DOT3/DOT4 reduces the number of items someone has to stock. It also reassures the driver that they used the right stuff, rather than worry that using DOT4 in their DOT3 brake system could be a problem. When I didn't know DOT4 could always replace DOT3, I felt good using Valvoline DOT3/DOT4 in my car, and in my parent's cars.

 

[buck91]

Originally Posted By: Johnny248

...Awhile back I looked into switching to 5.1 but for some reason I decided to stick with 4. I can't remember why. I'll have to go back and look at it again. For some reason something about corossion or water attraction sounds familiar as to why I didn't use it, but don't quite me on that.

A ten minute google and wiki search turned up a lot of info, most fairly general and a little outright false. Based on that search it looks like DOT 5.1 IS compatible in Dot 3/4 systems and other than the "trying a new thing" stigma I could not find any draw backs other than cost. Would love to learn more, though!

 

[JOD]

Originally Posted By: buck91

A ten minute google and wiki search turned up a lot of info, most fairly general and a little outright false. Based on that search it looks like DOT 5.1 IS compatible in Dot 3/4 systems and other than the "trying a new thing" stigma I could not find any draw backs other than cost. Would love to learn more, though!

The brakes on one of my bikes specs Dot 5.1 fluid. Availability and cost are 2 pretty good reasons not to use it! I think the Motul was $12.00 for 500ML. Ouch... Makes sense in a bike application where over-heating can be a real concern, but seems like overkill for most passenger cars.

 

[BHopkins]

Originally Posted By: Ken2

Quote:

why one wouldn't prefer to use 4 over 3 given the higher boiling point.

The same chemicals that give it the higher wet boiling point (borate esters) absorb moisture more readily. Changing it regularly is more important with DOT4 than with DOT3. I was in the local NAPA franchise store a while back, picked up a bottle of the synthetic DOT4 brake fluid and took it to the counter. The parts guy, an experienced clerk there who is very helpful, asked me if I was sure I wanted the synthetic version. I told him that it was all synthetic, and the label was just marketing hype. I'm not sure he believed me.

It has been my experience that many people, even professionals, confuse synthetic brake fluid with silicone brake fluid.

 

[cos]

Johnny, I hear you, besides Valvoline, I used to see Castrol and Prestone in the Detroit area. Now it's just Valvoline DOT 3&4, which I use, from Walmart or on sale from Advance.

 

[MolaKule]

Originally Posted By: Johnny248

Originally Posted By: MolaKule

All brake fluids are synthetic.

What's funny about that is that Valvolines product description of their Synthetic DOT3 / DOT4 fluid on their website says: •Compatible with all braking systems requiring a DOT 3 or DOT 4 fluid •Mixes well with conventional fluids Maybe I am reading that wrong, but seems to suggest they are misleading the customer into thinking that other fluids may not be synthetic. BTW, is product the same as the SynPower brake fluid? I haven't seen it in years so I was assuming they just changed the name.

It is not misleading but it could be confusing for the non-tech folks. They should have used the word "Standard" instead of "Conventional."

 

[MolaKule]

Originally Posted By: BHopkins

Originally Posted By: Ken2

Quote:

why one wouldn't prefer to use 4 over 3 given the higher boiling point.

The same chemicals that give it the higher wet boiling point (borate esters) absorb moisture more readily. Changing it regularly is more important with DOT4 than with DOT3. I was in the local NAPA franchise store a while back, picked up a bottle of the synthetic DOT4 brake fluid and took it to the counter. The parts guy, an experienced clerk there who is very helpful, asked me if I was sure I wanted the synthetic version. I told him that it was all synthetic, and the label was just marketing hype. I'm not sure he believed me.

It has been my experience that many people, even professionals, confuse synthetic brake fluid with silicone brake fluid. ALL BRAKE FLuds are synthetic.

You have to know the difference between the DOT fluids. If you don't, ask your mechanic to install it. Chemistry of Brake Fluid