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Old 11-14-2018, 06:43 PM   #1
darli328
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Default DOT 5.1 brake fluid question(s)

Everything I've searched or read has said DOT 5.1 brake fluid meets all the standards/requirements of DOT 3 and/or DOT 4. However my Subaru dealer is telling me DOT 5.1 is not approved and they won't change the fluid if I bring in my own DOT 5.1 (I made sure to specify DOT 5.1 and NOT DOT 5 when talking with the dealer). My 2017 STI is daily driven with the occasional spirited driving and I want to use Motul 5.1. I would like to track my car some day but I don't have any immediate plans to do so. Any reason why my Subaru dealer is telling me this? Am I missing anything regarding DOT 5.1? For my application would it be better to just get a good DOT 4 fluid? Thanks
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Old 11-14-2018, 06:47 PM   #2
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If you want to, that's on you. If it causes a failure, it's on you.


As long as you understand that.
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Old 11-14-2018, 07:50 PM   #3
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Quote:
Originally Posted by Hondaslayer View Post
If you want to, that's on you. If it causes a failure, it's on you.


As long as you understand that.
Could you be a more specific? Are you saying DOT 5.1 can cause a failure?
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Old 11-14-2018, 08:15 PM   #4
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I’ve read good things on ATE Type 200 brake fluid. Anyone suggest that over Motul 5.1 since ATE Type 200 is DOT 4 for spirited daily driving?
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Old 11-15-2018, 07:04 AM   #5
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I don’t see why your shop wouldn’t do that for you. DOT 3,4, and 5.1 are all compatible in terms of not messing up your brake lines/seals from my understanding.
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Old 11-15-2018, 07:11 AM   #6
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DOT 5.1 is compatible with 3 and 4.

Why are you using 5.1? Do you have a case of it sitting on a shelf and want to use it up? Do you think that somehow it's better than 3 or 4? It's not.

If this is a street car use, 3 or 4 or 5.1 are equal. And after a week of driving, they would morph to be 3. If you're going on the track tomorrow, then a very high dry boiling temp fluid is appropriate, but these fluids will absorb water very quickly, so should be changed out after the track session with a 3 or 4 type fluid.

It's always a good idea to look at parts and stuff you're using in your car. Using track brake pads on your car because you think it's "better" is simply wrong. What race type pads do is they don't fall apart under very high temps and hard use. But until they're up to temp, they do nothing. As in....you stand on the brakes and they don't slow you at all until they get up to full temp. With brake fluid, the higher the boiling point, the faster they absorb water. If you're having someone else change your fluid, just let them choose the fluid and do the work. That way, if you're driving down the street and stand on the brakes and the car doesn't stop, you go back and blame them. If you bring your own fluid, you know they'll point to the fluid and say "we warned you", whether it's the fluid's fault or not.
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Old 11-15-2018, 07:27 AM   #7
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OP, just get a DOT 4. I've used ATE Typ200 (and SuperBlue back in the day... RIP) and they were perfectly fine for every-day use and autocross, and yes even some track laps. Heck, even Motul RBF600/660 is DOT 4... BUT depending on what you're using your car for, keep an eye on those boiling points and pick a fluid accordingly.

Last edited by littledrummerboy; 11-15-2018 at 07:33 AM.
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Old 11-15-2018, 08:43 AM   #8
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Quote:
Originally Posted by WyoOutback View Post
I don’t see why your shop wouldn’t do that for you. DOT 3, 4, and 5.1 are all compatible in terms of not messing up your brake lines/seals from my understanding.
Correct, all 3 are compatible.
DOT 5.1 has a slightly higher boiling point, essentially the same as DOT 5 (which is NOT compatible).

Ideal is all one fluid type even if you can mix them.
You are better served by doing a flush every so often and as a last step at each wheel, open the bleeder and compress the piston to get the crap out of the bottom of the piston bore. I almost guarantee most peeps will get something that looks like Maple syrup out since the bleeder is above that.
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Old 11-15-2018, 01:22 PM   #9
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Quote:
Originally Posted by Jack View Post
DOT 5.1 is compatible with 3 and 4.

Why are you using 5.1? Do you have a case of it sitting on a shelf and want to use it up? Do you think that somehow it's better than 3 or 4? It's not.

If this is a street car use, 3 or 4 or 5.1 are equal. And after a week of driving, they would morph to be 3. If you're going on the track tomorrow, then a very high dry boiling temp fluid is appropriate, but these fluids will absorb water very quickly, so should be changed out after the track session with a 3 or 4 type fluid.

It's always a good idea to look at parts and stuff you're using in your car. Using track brake pads on your car because you think it's "better" is simply wrong. What race type pads do is they don't fall apart under very high temps and hard use. But until they're up to temp, they do nothing. As in....you stand on the brakes and they don't slow you at all until they get up to full temp. With brake fluid, the higher the boiling point, the faster they absorb water. If you're having someone else change your fluid, just let them choose the fluid and do the work. That way, if you're driving down the street and stand on the brakes and the car doesn't stop, you go back and blame them. If you bring your own fluid, you know they'll point to the fluid and say "we warned you", whether it's the fluid's fault or not.
Appreciate the info and everything you're saying about track pads makes sense. Years and years ago I use to auto cross an S2000 and then later a 350Z, back then I used Motul RBF 600 and never had any issues. Obviously Motul RBF 600 isn't a good idea for the street so I'm not trying use that for this application.
I somewhat wanted to stick with Motul products so I came across Motul 5.1 and now we're here. As you were saying with higher boiling points the faster brake fluid will absorb water and that's what I always thought as well. But, Motul 5.1 has a lower boiling point than some DOT4 fluid (Motul 5.1 boiling points dry 509F wet 365F and ATE Type 200 boiling points dry 536F wet 388.4F) and that made me think Motul 5.1 might be better for a street application than say ATE Type 200 DOT4 fluid. Am I wrong on boiling points compared water absorption rates with respect to DOT 3, 4 and 5.1 fluid? I'm really just looking for a quality fluid to use daily with some spirited driving and maybe an autocross/track use in the future. If I got serious about repeated auto crosses/track days I would swap out fluid as I would imagine others do. Your thoughts?
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Old 11-15-2018, 03:04 PM   #10
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Quote:
Originally Posted by littledrummerboy View Post
OP, just get a DOT 4. I've used ATE Typ200 (and SuperBlue back in the day... RIP) and they were perfectly fine for every-day use and autocross, and yes even some track laps. Heck, even Motul RBF600/660 is DOT 4... BUT depending on what you're using your car for, keep an eye on those boiling points and pick a fluid accordingly.
Did you use ATE Typ200 for daily driving? I’m looking for a good quality fluid that’ll last about 2 years/30,000 miles that preforms a little better than the stock DOT 3. I’ve read ATE Typ200 can last up to 3 years but I don’t plan on keeping the fluid in longer than what the owners manual says.
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Old 11-15-2018, 04:15 PM   #11
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I use Motul RBF600 for my DD. Call it overkill.
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Old 11-15-2018, 05:18 PM   #12
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Quote:
Originally Posted by darli328 View Post
Did you use ATE Typ200 for daily driving? I’m looking for a good quality fluid that’ll last about 2 years/30,000 miles that preforms a little better than the stock DOT 3. I’ve read ATE Typ200 can last up to 3 years but I don’t plan on keeping the fluid in longer than what the owners manual says.
Yes, I used it for dd and autocross for many years on my 06. iirc, I changed fluid every 2 years. I’ll be switching out my OEM fluid on this 17 with Typ200 next spring.

Last edited by littledrummerboy; 11-15-2018 at 05:24 PM.
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Old 11-17-2018, 11:38 AM   #13
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Anyone else have any thoughts before this thread gets buried?
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Old 11-17-2018, 10:29 PM   #14
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You're better off buying Walmart brand brake fluid and changing it out yearly as opposed to running some expensive brake fluid in a street car and never change it out very often cause is so race like and stuff.
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Old 11-17-2018, 11:38 PM   #15
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Quote:
Originally Posted by Scorpius View Post
You're better off buying Walmart brand brake fluid and changing it out yearly as opposed to running some expensive brake fluid in a street car and never change it out very often cause is so race like and stuff.
This guy gets it. My brake fluid is no name 99 cent a pint stuff. Unless you know who Nigel Mansell is without googling, you don't likely need blingy brake fluid.
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Old 11-18-2018, 09:58 AM   #16
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Just get some quality DOT4. You won't notice any difference. I guess Subaru just never tested the system with 5.1. I mean yes they are compatible but they are not the same. 5.1 does have lower viscosity so maybe it won't work as well as DOT4 in certain designs.
Do you know there are two kinds of brake systems? One is low pressure abrasive brake system used by europeans and one is high pressure adhesive brake system used by the japanese and america? These two systems have different caliper pressure so I would imagine a thinner brake fluid like 5.1 would cause some issues.

Last edited by jasonwrx86; 11-18-2018 at 10:07 AM.
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Old 11-18-2018, 01:06 PM   #17
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Quote:
Originally Posted by jasonwrx86 View Post
Just get some quality DOT4. You won't notice any difference. I guess Subaru just never tested the system with 5.1. I mean yes they are compatible but they are not the same. 5.1 does have lower viscosity so maybe it won't work as well as DOT4 in certain designs.
Do you know there are two kinds of brake systems? One is low pressure abrasive brake system used by europeans and one is high pressure adhesive brake system used by the japanese and america? These two systems have different caliper pressure so I would imagine a thinner brake fluid like 5.1 would cause some issues.
Helps to learn a bit about, "non-compressibility of fluids", frankly, an air free hydraulic system does not give a crap what fluid is used......until you add in water, air or high temp......

As to your comment on type of system, never heard that before, I (and others) would LOVE a link to decent supporting documents.....maybe we learn something......otherwise, I call BS in how brakes work.
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Old 11-18-2018, 04:07 PM   #18
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You want the fluid that absorbs moisture most readily, especially on a daily vehicle. The moisture will get into the system either way, from direct exposure to air in the reservoir, but the fluid that absorbs it quickest will also prevent water destroying the inside of your caliper bores. Rust can form around the circumference of a piston if moisture isn't absorbed by the fluid, and cause a sticking caliper. Moisture can also damage anything inside the brake system that isn't coated. Like others have said, use a standard DOT 3 or 4 fluid and flush it every couple years, more often if you track it.
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Old 11-18-2018, 05:11 PM   #19
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Quote:
Originally Posted by Charlie-III View Post
Helps to learn a bit about, "non-compressibility of fluids", frankly, an air free hydraulic system does not give a crap what fluid is used......until you add in water, air or high temp......

As to your comment on type of system, never heard that before, I (and others) would LOVE a link to decent supporting documents.....maybe we learn something......otherwise, I call BS in how brakes work.
Yes and no. Fluids of course is not compressible. However viscosity is depending on the tolerance of the parts. If an engine is designed to run 50W oil and you use 0W oil, the oil pump pressure can't keep up with the clearance in the bearings.
Same with engines that is designed to run 0W and you dump 50W in it, guess what the oil pumps can't flow the oil though the bearings to keep the bearings cool.
Same with brake fluids, DOT5.1 is half as thick as DOT4 and if the parts used in the brake system such as the ABS module is not designed for the reduced viscosity, it might cause performance differences.
And yes there are two kinds of brake systems. Adhesive and abrasive. Abrasive brake system uses abrasive brake pads with softer materail rotors so that it relies on the brake pads scrubbing materials off the rotors to achieve braking. My Mazda RX-8 for example used abrasive type of system. The rotors always look shiny, and they wore very quickly. There was an obvious lip on the rotor by 16000 miles of normal driving. I used brake pads that was designed for adhesive braking system such as the Hawk HPS and experienced significant braking performance issues. The freshly cut rotors showed very minimal scrubbing after 1000 miles with Hawk HPS. It had a coating of brake material like the adhesive brake system supposed to do but the braking system simply didn't have enough pressure for the pads to work properly. Once I switched back to Mazda pads, the cut marks on the rotors were gone in 300 miles and the rotors went back to the shiny look like before.
In comparison, the brakes on my WRX and Mustang are adhesive. The brake pads leaves a thin layer of pad material on the rotors and rly on the material adhesion to slow the car down. The benefit of this is the rotors will last a long time because the pads are softer.
The reason why europeans cars use abrasive braking is because abrasive braking has better high heat performance because it's a physical process. The bad thing is brake dust due to pad and rotor material being scrubbed off. You see a lot of BMWs and Mercedes have dirty front wheels just after a short drive is because this reason and they require rotor change far more frequently.
There is no standard brake system pressure across all manufactures so you will run into problems like what I had experienced with my RX-8. It was designed for metallic pads and when I go to organic pads, the braking performance was significantly reduced. Usually if you go the other way it's not an issue such as organic pad brake system switching to metallic pads.
There is a document you can find the information.http://www.brakeandfrontend.com/brakefriction/
And here is a link to my post regarding Hawk HPS on my RX-8. https://www.rx8club.com/rx-8-discuss...s-pads-258378/

Last edited by jasonwrx86; 11-18-2018 at 05:20 PM.
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Old 11-19-2018, 01:47 PM   #20
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Quote:
Originally Posted by Scorpius View Post
You're better off buying Walmart brand brake fluid and changing it out yearly as opposed to running some expensive brake fluid in a street car and never change it out very often cause is so race like and stuff.


That s what I do too.
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