Brake Fluid Military Testing - DOT 3 v DOT 4 vs DOT 5.1

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I have been researching brake fluid for a bit now to determine if a switch from DOT 3 to DOT 5.1 makes any sense for our vehicles. The one I'm most concerned about is our 2018 LEAF EV which calls for yearly fluid flushes, and has a very expensive integrated electronic braking system. We also see winter temps that can dip below -35C, so choosing a fluid with very low viscosity at low temps makes sense.

I found some nicely detailed and recent (2019) data from a study conducted by the US military to evaluate DOT 3, 4, and 5.1 fluid as an ABS friendly alternative to DOT 5 (silcone based) which does not work with ABS systems. The study really clears up some ambiguous information floating around on the interwebs.

Some key points:
  1. This research confirms more or less confirms that DOT 3 absorbs less water than 4, and DOT 4 less again than 5.1.
  2. They test EPDM vs SBD rubber parts/seals for swell/hardness and my interpretation there is that it's a wash.
  3. DOT 4 doesn't look so good in their tests.
  4. Corrosion results on the various fluids (other than 1 of the DOT 4 candidates) show excellent results on all the fluids.
  5. It looks like the DOT 5.1 will replace the silcone DOT 5 standard for the military.
  6. The DOT 3 LV (I believe this is Dow Brake Fluid 372LB) shows the best viscosity at very low temps (of all the fluids tested), the least water uptake, and very low corrosion numbers.
  7. The ? Bosch ES16-32N showed the highest boiling point for wet (4%) fluid.

I've attached two links...the second gives you an idea of the brands uses, but the study does not explicitly say which is which. I surmised that the DOT 5.1 C brand is the Bosch ES16-32N (DOT 5.1) based on its kinematic viscosity numbers at -40C. The DOT 5.1 E candidate may also be the BOSCH fluid, again just based on their spec of 680 mm2/s at -40C, compared to the measured results. This is what I'm standardising our vehicles to on a 2-3 year flush schedule. I'm using the Speedibleed pressure bleed system with correct caps for Honda, Nissan and Toyota...which works very well for a 1 person DIY setup with no two person pump/bleed required. I have tried vacuum at the calipers and it always required a pressure bleed to get a solid pedal..so I don't use this method any longer.

In any case, I hope this dispels some anecdotal myths floating around, and prompts some intelligent discussion on the matter :)

This is the study:


This is the market research on brands for the study. Note that a few of these manufacturers are bulk suppliers, so the actual brake fluid brand cannot be inferred. Look to the last page:



I look forward to your comments based on the study and perhaps some detective work on matching brands to the candidates.
 
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denwood

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I believe the DOT 3A LV tested is the same as this based on the MSDS:


The MSDS lists it as : Dow Brake Fluid 372LB which matches up with the DOT 3A LV candidate in the study (APPENDIX B - in 1070186)
This is listed as AC Delco GM OEM fluid.

If you're not racing, and need a DOT 3 which works very well at very low temps, this looks like your best bet. Even "wet" this fluid performed far better than the others with respect to low temp viscosity, which in turn should be the best for ABS and ESC system performance. However, it has the lowest boiling point both wet and dry.
 
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I was always at the assumption that DOT 3 absorbed water faster than DOT 4. I had assumed that based on previous Toyota's I've owned that specified DOT3 and a yearly change interval, vs other manufacturers using DOT 4 and specifying a 2yearly change
 
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I was always at the assumption that DOT 3 absorbed water faster than DOT 4. I had assumed that based on previous Toyota's I've owned that specified DOT3 and a yearly change interval, vs other manufacturers using DOT 4 and specifying a 2yearly change
Toyota has never specified an annual brake fluid change.
 
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I was always at the assumption that DOT 3 absorbed water faster than DOT 4. I had assumed that based on previous Toyota's I've owned that specified DOT3 and a yearly change interval, vs other manufacturers using DOT 4 and specifying a 2yearly change

Castrol North America had their LMA (low moisture activity) DOT 4 brake fluid. However, I've looked at a Toyota maintenance interval and as was noted they don't specify a change period, similar to power steering fluid.

I'm not even sure there's any hard and fast rule. The higher DOT standards only specify higher minimum dry/wet boiling temperatures and lower viscosity. They don't specify a requirement for moisture absorption, and I'm not sure that there's any hard and fast rule that DOT 4 has to absorb water faster than DOT 3. I believe there are just way too many variables. However, my understanding is that the particular composition needed for DOT 5.1 (high in borate esters) means it will tend to attract moisture faster than a typical DOT 3.
 
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I was always at the assumption that DOT 3 absorbed water faster than DOT 4.

That doesn't mean it's even relevant.

Given the OP's yearly fluid flushes it actually isn't. It also isn't given two or three
year intervals. I wouldn't keep any fluid regardless of spec longer than that anyway.
.
 

denwood

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Shane, on the DOT3 water absorption, yes, it's hard to find good data. But this study clearly shows that for long flush intervals, DOT3 is likely best.

Critic, ypw, I think the general rule from the brake fluid manufacturers of 2 years for a flush is not a bad idea. I do a full brake service at least once yearly on all our vehicles, so no seized calipers etc now in over a decade.

Adams, Bosch 5.1 could be candiate 5.1C or 5.1E. I wish I could figure out if it is E or not. In any case, the study should relieve any reservations about putting 5.1 in a DOT 3 spec car, as long as you're maintaining a 3 year flush interval. Shelf life on the Bosch 5.1 is 5 years, and is a good deal from Rockauto.

930, agree 100%. 2-3 years is the max I leave brake fluid without a flush now. It's like 20 minutes with a Speedibleed while the car is up on the hoist for winter/summer tire/rim swaps. Had I seen this study earlier though, I'd just get that AC Delco DOT 3 for the cars as we're not in the mountains, not towing heavy trailers, and not descending mountains :) That DOT 3 is the best of all of them for low temp operation of ABS and ECS. The military suggests kinematic viscosity in the 2000 mm2/s is dangerously thick, but the DOT 3 LV is easily the winner at -40 C (dry and wet) in the 600 mm2/s (dry) and 900 mm2/s (wet) range. All of the 5.1 fluids were better than spec DOT 3 (not LV) fluid though.

I've yet to see any studies with regard to ABS/ESC performance with cold/thick fluid, but I certainly know what it feels like at the pedal (very stiff) in those extreme temps. It's also at these temps that conditions demand more of ABS and VSM, particularly with our EV pulling regen braking from the front tires only. The braking computer has to dial in rear brakes (aside from ABS) to counter wheel spin incurred during aggressive regenerative conditions where the front tires are seeing high deceleration. That can be a recipe for disaster in a front driver, so it's a good thing the VSM system is so good :)
 

denwood

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Denwood, thanks for sharing. Very interesting. For the most part, the study confirms what has been discussed and learned before, here on BITOG. But it is great to have that data now, as a solid resource for naysayers.

I found the risk assessment in Table 10 very interesting. While many may find DOT 3 to be a good choice because of slower moisture absorption rate, the Army gave DOT 3 a High Level of Risk, primarily due to how it scored for Vapor Lock Prevention. I suspect this is related to the low ERBP (Equilibrium Reflux Boiling Point), that they noted as the primary reason why DOT 3 will not proceed for further consideration.

Considering that 1) Even though DOT 4 and DOT 5.1 fluids absorb moisture faster than DOT 3, even after high (4%) moisture absorption they still have significantly higher boiling points than DOT 3, and 2) All the fluids passed the corrosion test at 5% water, I would tend to think that a DOT 5.1 is the best choice. Particularly for anyone that does any regular brake fluid change.

It does seem that DOT 3 would be a good choice for someone who's driving style is very, shall we say subdued. No hard braking. No pulling trailers. No steep declines.

But, for me at least, it is hard to understand the cost argument for staying with DOT 3. The Bosch DOT 3/ DOT 4/ DOT 5.1 fluid can be bought for about $17. Members here on BITOG will spend good money to buy synthetic over dino oils, Castrol or Mobil 1 over store brands, Fram Ultra, Mobil, or OEM filters over jobber filters. But then they bicker about spending an extra $10 every year or two on brake fluid.
 
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denwood

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@BHopkins , yes the page 10 chart excludes the DOT 3 LV, however if the weighting for vapor lock is removed, it is the winning candidate.

I have only encountered brake fade on the street (never had the issue autocrossing) once, and it was not fun. My interest in the study was in finding a brake fluid that would be "safe" for our 2018 LEAF EV, which specs DOT3. It also has a very expensive electronic brake controller integrated to the master cylinder. The data in this study pretty much puts the seal swell/degradation issue between DOT3/4/5.1 to bed, and confirms that the 5.1 (or the DOW 373LB low viscosity DOT 3) fluids would be excellent choices for cold weather performance. There are likely other brands, but the "AC Delco DOT 3 Brake and Clutch Fluid" was the only one that I could confirm was actually DOW 372LB...the low viscosity DOT 3 used in the test.

For someone concerned about straying from an OEM DOT 3 recommendation and not flushing regularly, I'd for sure use the AC Delco DOT 3, otherwise, the Bosch 5.1 is only 8$ USD per quart at Rockauto. I'm standardising our three vehicles to the DOT 5.1 so I only have to keep the Bosch product on hand. They claim a 5 year shelf life so you can stock a few quarts in your shop.

With the cost of the components in our "cheap" EV, you're looking at $600 for new rear caliper with electronic parking brake integrated, and $4000 for a new brake controller. Ignoring brake fluid in these cars is an expensive proposition...and I suspect Nissan is aware as they recommend yearly brake fluid changes!
 
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I have only encountered brake fade on the street (never had the issue autocrossing) once, and it was not fun.

Do you know why this happened? :unsure:

On a sidenote, it's very important to not mistake 'brake fade' (due to over-
heated brake pads) with 'loss of brake pressure' (due to boiling brake fluid).
.
 

denwood

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Yes, I overheated the brakes on my autocross car..which was on the street at the time. Four hard stops on a long descent. It was likely older DOT 3 at the time and yes, it was likely fluid boil as the pedal went to the floor. That was resolved with a fluid flush and pad/rotor upgrade. DOT 5.1 didn't exist at the time as this was likely 20 years back. I remember it quite clearly though as when the pedal goes to the floor, your adrenaline will pump a bit..ha :) Call it a lesson learned.

Fade with heat = solid pedal but poor/no braking.
Loss of braking due to vapour/boil = soft pedal, poor/no braking.
 
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i haven’t experienced brake fade in 30 years, including mountain towing with brakes so hot they were smoking. Rotor cooling seems to be working well. That said, i do tend to change our brake fluid about every 3-4 years, so it’s by no means neglected.
 

denwood

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i haven’t experienced brake fade in 30 years, including mountain towing with brakes so hot they were smoking. Rotor cooling seems to be working well. That said, i do tend to change our brake fluid about every 3-4 years, so it’s by no means neglected.
I've only run into once (going back about 20 years) and only on that autocross car. That said, it was four hard consecutive stops, going downhill on the street. Autocross itself, never had an issue. That car (84 Scirocco) also got quite a few upgrades over the years, including rear discs etc. to match the larger motor and race suspension.

Regardless, now that we're driving a car with a $4000 electronic brake controller, and recommended yearly flushes, I'll for sure be following that schedule with the Bosch ESI6.
 
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