I am working on hydraulics these days and am looking to pick a brake fluid to go with. As my car is freshly painted, I am naturally not too thrilled about conventional DOT 3/4 brake fluids being near my car, if I can help it.

For now, my car will be a primarily street car, some spirited driving and the occasional autocross event. I do not have plans for heavy, high-speed/repeated , braking at this time [road course].

I would like to use DOT5 in my hydraulic clutch and brake system and am curious to hear your experiences.

Wilwoods DOT5 has a 500°F boiling point. Their best DOT4 is about 600°F.

I like Cartel brand Dot5. It doesnt seem to foam up like many others I have tried. That said I have not tried Wilwood Dot 5

http://www.castrol.com/en_au/australia/products/cars/brake-fluids/react-srf-racing.html

Only brake fluid I ever use, even in my S2000 street car which is a rare blue color.
Brake fluid takes time to strip paint and unless you spill it all over it should not be a problem

I wouldn't talk you out of it. On a new build with all new lines and fresh paint, pour it in. If you have a leak no peeling paint while you tighten up the fittings. If you later decide to go balls out on a road course, once you know there are no leaks, swap it out for some race stuff.

Whenever I bleed a fresh system I keep a squirt bottle of Simple Green handy. Any spills or leaks are vigorously sprayed.

I have put DOT 5 in 2 new builds and I will use it in the next one. Of course they had all new parts.

Motul, just be careful and rinse it off quick if you spill.

Demon got all new cylinders, lines, prop valve and master cylinder when I had it apart. Been using Kleen Flo DOT 5 since then (2007) with not a single issue.

Car of course brakes much better after changing out the tiny drums it came with, so I can't speak to any potential performance differences between the fluids.

I first used dot 5 in 1981, and it is so awesome, I always use it in any thing that matters ever since.
I put it in my 442 in 81, drove it untill the end of 1987, stored it for 23 years, then drove it on power tour
Without changing fluid or touching the brake system. Try that with dot 3!

Pretty sure I wouldn't try that with anything.

I had heard that moisture in the system will turn dot 5 to hard solid silicone. No truth to that? All brake systems pick up moisture over time, which is why they rust from the inside too, and fluid turns dark brown or black without regular maintenance.

See if you can find anything in print (including internet) that supports that claim.

If you want my guess, it sounds like something posted in sarcasm that was then repeated as fact.
First of all, the DOT designations are part of the FMVSS - Federal Motor Vehicle Safety Standards. They're there to protect us (and the manufacturers) on critical items. So a fluid that turned solid making the brake system completely useless would never have made it into the standard except as a major F-up.
Second, silicone based (primarily dimethyl polysiloxane fluid) was created in large part because it is not hygroscopic.
For that reason FMVSS 116 (Brake Fluid standards) test procedures for DOT 5 relating to water absorbtion are different than for DOT 3 and DOT 4.

FWIW, SAE J1704 is where the composition of the different brake fluids is described. That's an industry standard rather than a Federal standard. The Feds only care if it works, not how it works.

As a result, DOT 4 usually has esters as well as glycol. It mixes with DOT 3 and has similar viscosity. Both absorb moisture over time. Both should be changed as needed; If racing, the moisture lowers boiling point and increases compressiblity. If just regular or weekend driving; the corrosion inhibitors get used up and rust can become a problem.

DOT 5 silicone based has several characteristics that make it unsuitable for certain uses. The viscosity doesn't work well with some ABS. In some environments, moisture that does get in the system could (theoreticly?) collect in low points. (I suspect if this was a real issue, we'ld hear and read anecdotal stories about it). Finally it is more compressible than the glycol and ester fluids. This compressibility goes up with temperature.

Road racing requires every bit of pedal feel and braking performance possible. Road courses also require repeated braking that can elevate brake fluid temperatures far higher than seen on most public roads or other competition venues. A pedal that gets spongier and travels further during a race is at best disconcerting!

One public road situation where brake fluid can get pretty hot is long and repeated downhills with heavy braking loads. So yea if you're driving your classic in places where the brakes get hot enough you can smell 'em, DOT 5 may not be the best choice there either.

So to the original question by the OP, for many situations DOT 5 is fine. Yours seems like it would good for what you want and plan to do.

I dunno what's with the Aussie brake fluids being linked, how about y'all kink a North American source?

Aussies seem to do motorsports better than us Yanks, burnouts too!
One can easily get Castrol SRF or Motul from Summit, Amazon, major race tracks, etc.

The good stuff doesn't come cheap though!

I concur with most of your other points, but I have "read" the compressibility issue with DOT5 is, it traps/entrains air when pouring, and first impressions just after installing, leads to the compressibility observations, unfairly, if confirmable. The rising temp issue leading to increased compressibility, is the first I have heard of that condition.

Any car with DOT 3 that I have left sit for more than a couple years has no pedal when I get it out of storage. The fluid has absorbed moisture and rusted up the cylinders. Minnesota is damp. My experience related in the post above positively demonstrates that DOT does not attract moisture and ruin the brake system. For those of us who store our cars over the winter or longer, this is vitally important. We don't want to rebuild the hydraulic system of the brakes every other year. All of the internet hype about the drawbacks of DOT 5 is irrelevant to 95% of users, including me and the OP.
In 2002 I bought a set of Wilwood front disc brakes for my race car. They had a tag warning not to use DOT 5. I called them and asked why. They said DOT 5 is not hygroscopic, so moisture will collect in the brake system and it will fail. I called bs on that and put in DOT 5. 14 years later I still have those brakes and they have not yet failed or had any issues. Wilwood has since changed their tune, and now they sell DOT 5 with their own name on it, they still say don't put it in a race car. I suppose in another 14 years they will sell race car DOT 5.

Thanks for the feedback all. I went ahead and bought DOT5 and have gotten the clutch out of the way thus far. Even if I were to swap it out every year for moisture, the peace of mind is worth it.

I use DOT5 in my stuff too with no problems. It's nice not worrying about it when it gets on the backing plates and painted calipers when bleeding too.

A reason for the assertion of compressibility is that if dot 5 container is not handled very gently, that very fine air bubbles can be produced in the fluid that will cause this problem as air is compressible. The key is to not shake the container and if it has been do not use until fluid is again clear. I have no idea how long that might take.

I just recommend what handles actual use the best. I dont really care what is best at sitting still. People out there beating the snot out of their cars lap after lap, I can pretty much promise do not use DOT 5.

I know this is an older post, but I heard of the bubble issue too. I read somewhere to put container of brake fluid in a pan of boiling water just before use. The warmer temperatures will cause the smaller bubbles to expand and float to the top. Handle gently after wards once it cools.

I tried that trick once to get air bubbles out of Dot 5 and just about killed myself. The Dot 5 gives off some powerful fumes when it is hot and I haven't to get a chest full of the fumes. Not a good deal at all.

These days I stick with a high quality non Dot 5 brake fluid and change it on a regular basis. I find that if I flush the brakes every couple of years then there isn't any rust and pedal stays nice and firm.

I don't recall fumes being getting too strong. Then again, maybe my memory is fuzzy because of the fumes. But I heated up a pan of water and sat container in the water. Never got above boiling point of water, so maybe that wasn't warm enough.

I would think a fluid designed for high temp (500F+?) use would be rather limited in ANY fume discharge, as fumes indicate an increase in evaporation of a fluid component, ie bubbles? I suspect another unknown factor in the Andy choking experience.

I'm using Dot 5 with no issues so far.

Air can get in a system when servicing (replacing) components.
With respect to compressibility, introduced air bubbles was not the characteristic I was referring to.

If air is entrapped in the system (bubbles) this will of course be noticible when the brakes are pushed hard because air is far more compressible than hydraulic fluid.

It appears that the reason silicone based fluids are more compressible relates to the density of the fluid. Further, that the dimethyl polysiloxane contains significantly more disolved air than glycol ether based fluids. This is air between the molecules and does not effect volume.

This dissolved air is not something to generally be concerned about. The problems for racers is twofold. First, they tend to be very sensitive to changes in pedal feel and vehicle response. Second, the brake fluid can get hot, very hot. The silicone fluids get noticibly more compressible as the temperature rises. This is disconcerting to say the least.

For making a decision about whether this may be a concern here's something to consider. If the pads and shoes aren't getting that hot, then the brake fluid isnt going be very hot. So the first question is whether the car will be in situations that require linings for higher temperatures and/or use up pads each event.

ref. Air solubility information comes SAE J1705, Appendix A, A.2.2.8 which was quoted in two unrelated sources. One a manufacturer's promotional paper The ABCs of Brake Fluid and the other in appendix A of a 1992 US Army investigation Purdy, Ellen Report 2505 (pdf)

The practical effect of compressibility is covered in the army report but does not cover situations where the fluid temperatures get elevated. A 1981 SAE paper given by G W Holbrook has been referenced for testing that showed that gycol fluids compressed less than .3% from 77 to 250 degrees F. Whereas silicone fluid compressed .85% at 77 F and had almost doubled that to 1.54% at 250 F.
At 400 degrees, the silicone compressed 2.41% while the glycol was around 0.5% A couple of important points here. If the glycol based fluid is old, it will have absorbed moisture and its boiling point could be under 400 F. Second is that depending on the braking system, the compressibility may be noticible not just in feel but in pedal motion. Holbrook reported additional 1/2" of pedal movement at 400 F. The reporter (who clearly prefers DOT5) calculated increased pedal movement for his triumph at different conditions. He came up with very just 1/10" with new pads at 150 F, to 0.4" with worn pads when at 440F.
One thing every source seems to agree on is not to mix the two types of fluid!

Very interesting, however it is hard to fathom this whole issue revolves around a 2%? difference of compressiblity at 400F?, if I digested your details correctly, and I have little knowledge on the this subject to dispute them in the first place. Would not be the first time we get lost in the weeds.

No one has to talk you out of anything.
Dot 5 is not allowed in most high performance driving schools and road racing venues. When it gets hot the brake pedal goes to sponge.
It's a poor choice in the handling forum. Period
I don't care of about analalfie and his whatever car and his braking data. As usual advice is only good if you listen to it.

Dude, You're the guy who asked. No one is lost in the weeds but you. You can't fathom the pucker factor when your foot goes another 3/4" at 100 mph? Really?

Yes, really, I only push hard enough to accomplish the task, and if one is so set in driving our non F1 cars that the idea of pushing 2% more fluid volume is a huge problem, you might want to stay in the weeds (every turn, every second, every time, every wind shift, any fuel consumption, etc requires a new/different braking pressure as conditions vary) (I have no idea where your 3/4" number comes from) (and if you are getting 400F fluid temps, you are on the proverbial edge in more ways then just fluid temps) ( a sudden 400F fluid temp rise would be a rather unusual set of circumstances) Dude.

We use DOT 5 in Postal vehicles(the old ones) We have never had a problem with corrosion,air or bubbles. I use it in my cars with no problems.

not sure why but fluid for ford's have a higher temp rating so I usually buy these for DOT3
http://www.canadiantire.ca/en/pdp/oem-dot-3-brake-fluid-ford-500-degrees-946ml-0381928p.html#srp

DOT 5 is the way to go with fresh paint. Problem I ran into is the stuff is hard to bleed out with fresh lines and getting a good seal at the fittings.

You should be using DOT4 in a performance car. You can replace DOT3 in any vehicle directly without worrying about having leftovers.

I use ATE Typ200 which has a 534 F boiling point. It also has good anti-corrosion properties. Spec'd by people who manufacture OEM brake calipers.

I am not aware of any OEM vehicle that comes with DOT5. My job is working on ESC modules and we don't allow DOT5 with our parts whatsoever.

Avanti, Harley Davidson, probably some exotics.

Dot 5 or Dot 5.1?

Dot 5 isn't compatible with ABS systems so I doubt any OEM is using it.

5.1 is NOT silicone based.

For what it's worth, for street use I haven't had any issues with the DOT5, but it's also only been 1,500 miles.

....the Wilwood pad knockback, however, is a different story.

(Have had DOT 5 in a street car for over 20 years...

20,000+ miles...

In all kinds of conditions...

Zero issues...

Wouldn't even consider going back to 3...

Goody,
Is that knock back on fronts, rears or both?

Thanks, Wes

Haven't quite determined that just yet.

If there is any ABS/ESC on these vehicles (which is required on USA sold cars since 2011), I can pretty much assure you they aren't using DOT5 silicone brake fluid as I'm not aware of any supplier that allows it. I worked at 2 of the 6 major companies that sell these systems. DOT5.1, they allow but I don't consider that to be very different than DOT4.



Thank you, I believe you are correct.



What's your rotor runout? If they are the two piece hub type, perhaps you can re-clock the rotor on the hub to help it. IMO over 40 micron is too much on anything near a new condition.

also from wilwood's own site:
http://www.wilwood.com/TechTip/TechBrakeFluidTip.aspx

Gosh, I just love this topic. And I always did.

Haven't bothered measuring runout as it only appears after a decent turn. I suspect it's more of a dynamic problem. I will be investigating a residual valve, which has been recommended across a few resources I have come across.



Understood. When I consider racing the car, I will flush out the DOT5. Until then, I have had no issues with as aggressive of street braking as I can get.

pad knock back is typically from loose wheel bearings...how did you set up the wheel bearings?
...and dot 5 in a track type street car is amusing...

Wheel bearings were setup to factory spec, actually tightened slightly, no improvement. Rears are green bearings, as required with the Wilwoods.

My friend in the Army that is a heavy vehicle mechanic says all the vehicles they service run DOT 5 fluid.

That would correspond with the Army Report I cited (and linked) on the previous page.

All of these fluids have an application.

Some people assume that posts in this subforum are oriented toward racing, and on the other side, there are a few who don't have clue that some people are here because they are very interested in high performance driving and racing. Otherwise the pros and cons seem to be outlined pretty well. Both silicon and non-silicone have their place.

Whichever you choose, it will depend on the intended uses and priorities.

Good write up Matt.

Also I remember reading about problems back when GM Started using Silicone . Something about it causing the rubber components swelling up to some degree and causing problems.

From my brief experince with road racing, If you are having a problem boiling DOT 3 then you can find problems elsewhere in your brake system, ie, Old Caliper piston seals that without elasticity cause the pads to drag and generate more heat, inefficient calipers such as floater type or a weak design that flexes with increased temp, not enough air flow. Pads installed without enough clearance on backing plates to move freely, etc, etc

The brake fluid compatibility standard, S5.1.10, of 49CFR571.116 exempts DOT5 from the requirement not to stratify.
"(2) Fluids, except DOT 5 SBBF, shall show no stratification."

Further the manufacturers themselves discouraging mixing.
eg https://www.penriteoil.com.au/assets/pdf/tech/Nov2015/Brake_Fluids.pdf

Anyone switching should flush the glycol based fluids out thoroughly. Good paper referencing SAE papers and standards as well as the usually suspects from Carol Smith to various manufacturers and retailers. http://www.niagarabritishcarclub.org/index_htm_files/Brake%20Fluid.pdf

I stand by what I wrote before. Its a case by case personal decision. I personally wouldn't use SBBF in my car as I want everything in my favor on road course type use. For someone with a restored car with all new or cleaned hydraulics, its a rational choice. For road racing, of course there may not be a choice - follow the rules of the sanctioning body and host track.

I find this discussion very interesting. One thing I would caution is something that has not been brought up as of yet. Any silicone based product is a nightmare to deal with if you every have to repaint any portion of your car.

In other words if any of the silicone gets on a painted surface it is almost impossible to remove fully and will cause fish eyes in future paint application.

That sticker would suggest that they don't want you to mix it.
LOL