I Decided on DOT3, Am I Wrong?

[RyanY]

Either type will work just fine in a Honda.

I start looking for DOT3 and DOT4 sales about 6 months prior to the scheduled fluid replacement, and the first one I find is what goes in. So far I have used DOT3 once and DOT4 once (3 yr increments), and haven't been able to tell a difference in performance.

This summer will be the end of the third 3 yr interval and I will install DOT4 Prestone from a recent Walmart clearance!

 

[tshephard]

Pff... the only DOT3 I've ever bought is Supertech. DOT3 is DOT3, bros. If the car came with it and you aren't racing it... generic DOT3 for me.

This was true for me... this week my Durango needed a cylinder and all my WM had was ST DOT4.

I may have to shorten my normal 22 year cycle.

 

[y_p_w]

I wouldn't specifically worry about it, but I've generally asked for DOT 4 or even DOT 5.1 sometimes. There isn't any specific requirement for exact composition. All DOT 3/4/5.1 has to be compatible with each other. DOT 5 is silicone and not compatible with glycol brake fluids. The number only specifies minimum performance for certain requirements and doesn't specifically specify how much it attracts water. Castrol used to sell their DOT 4 brake fluid as "LMA" for "low moisture activity". I've seen some higher performance DOT 4 brake fluids including the old Valvoline SynPower brake fluid which had specs approaching DOT 5.1 performance.

There are also minimum standards for viscosity based on DOT level. DOT 5.1 is required to have the lowest viscosity. However, if something meets the DOT 5.1 requirement for viscosity but only the DOT 3 requirement for wet boiling temperature, it's classified as DOT 3. DOT 5 numbers apply to DOT 5.1, although it's not silicone.

The specifications are all here:

https://www.govinfo.gov/content/pkg/CFR-2011-title49-vol6/pdf/CFR-2011-title49-vol6-sec571-116.pdf

​

S5.1 Brake fluid. When tested in accordance with S6, brake fluids shall meet the following requirements:​

​

S5.1.1 Equilibrium reflux boiling point (ERBP). When brake fluid is tested according to S6.1, the ERBP shall not be less than the following value for the grade indicated:​

(a) DOT 3: 205 °C. (401 °F.).
(b) DOT 4: 230 °C. (446 °F.).
(c) DOT 5: 260 °C. (500 °F.).​

S5.1.2 Wet ERBP. When brake fluid​

is tested according to S6.2, the wet ERBP shall not be less than the fol- lowing value for the grade indicated:​

(a) DOT 3: 140 °C. (284 °F.).
(b) DOT 4: 155 °C. (311 °F.).
(c) DOT 5: 1 180 °C. (356 °F.).​

S5.1.3. Kinematic viscosities. When​

brake fluid is tested according to S6.3, the kinematic viscosities in square millimeters per second at stated tem- peratures shall be neither less than 1.5 mm2/s at 100 °C. (212 °F.) nor more than the following maximum value for the grade indicated:​

(a) DOT 3: 1,500 mm2/s at minus 40 °C. (minus 40 °F.).​

(b) DOT 4: 1,800 mm2/s at minus 40 °C. (minus 40 °F.).​

(c) DOT 5: 900 mm2/s at minus 40 °C. (minus 40 °F.).​

I've heard of racers using "heavy duty" DOT 3 brake fluid with a high dry boiling point (typically 450+ºF). As long as it's flushed often (a racer might do it before a racing weekend) the wet boiling point will never be an issue. That's actually higher than the minimum dry boiling point for DOT 4.

 

[Jetronic]

I think all brake fluids must be synthetic.
Not like Dino engine oils.

All DOT 3, 4, 4 LV, 5 and 5.1 brake fluids are synthetic. But there's also mineral brake fluid.

 

[edyvw]

I wouldn't specifically worry about it, but I've generally asked for DOT 4 or even DOT 5.1 sometimes. There isn't any specific requirement for exact composition. All DOT 3/4/5.1 has to be compatible with each other. DOT 5 is silicone and not compatible with glycol brake fluids. The number only specifies minimum performance for certain requirements and doesn't specifically specify how much it attracts water. Castrol used to sell their DOT 4 brake fluid as "LMA" for "low moisture activity". I've seen some higher performance DOT 4 brake fluids including the old Valvoline SynPower brake fluid which had specs approaching DOT 5.1 performance.

There are also minimum standards for viscosity based on DOT level. DOT 5.1 is required to have the lowest viscosity. However, if something meets the DOT 5.1 requirement for viscosity but only the DOT 3 requirement for wet boiling temperature, it's classified as DOT 3. DOT 5 numbers apply to DOT 5.1, although it's not silicone.

The specifications are all here:

https://www.govinfo.gov/content/pkg/CFR-2011-title49-vol6/pdf/CFR-2011-title49-vol6-sec571-116.pdf

​

S5.1 Brake fluid. When tested in accordance with S6, brake fluids shall meet the following requirements:​

​

S5.1.1 Equilibrium reflux boiling point (ERBP). When brake fluid is tested according to S6.1, the ERBP shall not be less than the following value for the grade indicated:​

(a) DOT 3: 205 °C. (401 °F.).​

(b) DOT 4: 230 °C. (446 °F.).​

(c) DOT 5: 260 °C. (500 °F.).​

​

S5.1.2 Wet ERBP. When brake fluid​

is tested according to S6.2, the wet ERBP shall not be less than the fol- lowing value for the grade indicated:​

(a) DOT 3: 140 °C. (284 °F.).​

(b) DOT 4: 155 °C. (311 °F.).​

(c) DOT 5: 1 180 °C. (356 °F.).​

​

S5.1.3. Kinematic viscosities. When​

brake fluid is tested according to S6.3, the kinematic viscosities in square millimeters per second at stated tem- peratures shall be neither less than 1.5 mm2/s at 100 °C. (212 °F.) nor more than the following maximum value for the grade indicated:​

(a) DOT 3: 1,500 mm2/s at minus 40 °C. (minus 40 °F.).​

(b) DOT 4: 1,800 mm2/s at minus 40 °C. (minus 40 °F.).​

(c) DOT 5: 900 mm2/s at minus 40 °C. (minus 40 °F.).​

I've heard of racers using "heavy duty" DOT 3 brake fluid with a high dry boiling point (typically 450+ºF). As long as it's flushed often (a racer might do it before a racing weekend) the wet boiling point will never be an issue. That's actually higher than the minimum dry boiling point for DOT 4.

450f fluid might cut it in autocross but on track most likely not. Track race fluid used are all above 570f.

 

[nthach]

I think all brake fluids must be synthetic.
Not like Dino engine oils.

Yes and no. DOT fluids are synthetic glycol based for the exception of DOT5, a silicone fluid.

Some cars - like older Rolls-Royce and Bentley products(before they got switched over to BMW and Audi platforms respectively), some older Jaguars and road/mountain bike disc brake systems from Shimano/Magura/Campagnolo require mineral oil(Castrol HSMO or a specific Shell/Castrol hydraulic fluid), and Rolls-Royce/Bentley have a special Castrol RR363 fluid which is called for on certain models.

 

[ad244]

Thanks for all the helpful comments, I will sleep well at night.

I will say, the brakes perform awesome regardless.

 

[JHZR2]

Bringing this thread back because I thought I had read that interchanging dot4 for 3 in some systems could affect pedal feel. Similar to how dot4 vs dot4 lv can do so.

Any truth to that?

 

[slacktide_bitog]

Bringing this thread back because I thought I had read that interchanging dot4 for 3 in some systems could affect pedal feel. Similar to how dot4 vs dot4 lv can do so.

Any truth to that?

usually air in the system after bleeding. Speed bleeders are awesome

 

[Hall]

Bringing this thread back because I thought I had read that interchanging dot4 for 3 in some systems could affect pedal feel. Similar to how dot4 vs dot4 lv can do so.

Never heard much about pedal feel differences between DOT3 and 4 but 4 LV, yeah. Wife's Fusion specs DOT 4 LV so I've got Pentosin DOT 4 LV on hand plus we've flushed/exchanged the son's '12 Accord with that same fluid.

 

[JHZR2]

usually air in the system after bleeding. Speed bleeders are awesome

Which I have but still have a soft pedal in a gm… but I “upgraded” to DOT4.

 

[Tifosi]

My understanding that brake fluids are forward compatible but not backward. As in if the manual/brake system calls for DOT 3 fluid then DOT 3, DOT 4 and DOT 5.1 can be used. However, if the DOT 4 is required, you can not use DOT 3 but can "upgrade" to DOT 5.1. The only exception to this rule is DOT 4 LV as in if the particular ABS system requires DOT 4 LV then it is the only fluid that can be used.

 

[Hall]

The only exception to this rule is DOT 4 LV as in if the particular ABS system requires DOT 4 LV then it is the only fluid that can be used.

Don't hold me to this, but in the case with Ford, they recommend DOT 4 LV but DOT 4 is okay. Pentosin says you can mix the two as well with no issues but I think they mean you can ADD LV fluid to a DOT 4 system (since they're "promoting" LV).

 

[Tifosi]

Don't hold me to this, but in the case with Ford, they recommend DOT 4 LV but DOT 4 is okay. Pentosin says you can mix the two as well with no issues but I think they mean you can ADD LV fluid to a DOT 4 system (since they're "promoting" LV).

Learn something new everyday, thanks.

One of them being, don't buy a Ford.

 

[JHZR2]

My understanding that brake fluids are forward compatible but not backward. As in if the manual/brake system calls for DOT 3 fluid then DOT 3, DOT 4 and DOT 5.1 can be used. However, if the DOT 4 is required, you can not use DOT 3 but can "upgrade" to DOT 5.1. The only exception to this rule is DOT 4 LV as in if the particular ABS system requires DOT 4 LV then it is the only fluid that can be used.

Don't hold me to this, but in the case with Ford, they recommend DOT 4 LV but DOT 4 is okay. Pentosin says you can mix the two as well with no issues but I think they mean you can ADD LV fluid to a DOT 4 system (since they're "promoting" LV).

Not concerned about compatibility. The question becomes if the different fluids affect pedal feel, soft pedal, etc.

 

[Tifosi]

Not concerned about compatibility. The question becomes if the different fluids affect pedal feel, soft pedal, etc.

The answer can be highly subjective and can be convoluted by other factors. I have seen people saying the pedal feels firmer with increase in DOT value, the argument is based on different compressibility of the fluids. I have also seen people to say otherwise. I have yet to seen an actual test on this precise subject at this time.

 

[930.engineering]

My understanding that brake fluids are forward compatible but not backward. As in if the manual/brake system calls for DOT 3 fluid then DOT 3, DOT 4 and DOT 5.1 can be used. However, if the DOT 4 is required, you can not use DOT 3 but can "upgrade" to DOT 5.1. The only exception to this rule is DOT 4 LV as in if the particular ABS system requires DOT 4 LV then it is the only fluid that can be used.

You could also use a DOT4 or DOT5.1 when the manual calls for a DOT4LV,
if you aren't running that car in particularly cold environments. Brake fluid
viscosity depends on temperature just like oil (even it's thinner in general).

Btw, I really don't think pedal feel is a matter of whether you use DOT3, DOT4
or DOT5.1, it's likely just some air left.
.

 

[Hall]

Not concerned about compatibility. The question becomes if the different fluids affect pedal feel, soft pedal, etc.

You'll really never know for sure. New fluid, whether it's DOT 3, 4, 4 LV, or 5.1, will feel different (better ??) than older fluid.

 

[MolaKule]

I think all brake fluids must be synthetic.
Not like Dino engine oils.

 

[TacoFergie]

Out of curiosity regarding different DOT ratings and viscosity. I have a 91 Toyota MR2 NO ABS that specifies DOT 3 for brakes and clutch systems.

Is there a fluid that has better lubricity than others for the clutch system?

Also since it is almost 30 years old is there any brake fluids I should steer clear of and I am curious if there are any advantages to an LV fluid in either the brake or clutch systems?