Brake Fluid - Devils Advocate

[RogerH]

Hi Folks,

I have just spent a good hour looking at topics regarding facts about brake fluid. There is some utter tosh out there.

What have I found -

DOT4 (PEG) type fluids are good paint removers

DOT5 (SiliconE) aerates but does not remove paint.

These are facts and that's about it.

Often on forums - this one included - statements are made that are not actually factual. They may be good practice but that is all.

So I have come up with a few questions for anybody to support or otherwise with FACTS.

Q1 – is there definitive proof/information that you  must flush every last drip of brake fluid out of a system if you are changing from DOT4 to DOT5 or reverse.

Q2 – is there definitive proof/information that you must change the rubber components (seals, pipes etc) when changing from DOT4 to DOT5 or reverse.

Q3 – is there definitive proof/information regarding the swelling of rubber and that DOT4/5 cross contamination will cause major problems – excess swelling etc

Q4 – is there definitive proof/information that cross contamination between DOT4 & 5 will damage rubber components.

 

Regarding Q1 - clearly DOT4 remaining in a DOT5 system may lower the temp spec. But if the remnants is say 0.001% then what affect will it have - none I would suggest.

But just like this scenario I have no proof.

 

Over to you guys to provide the supported FACTS

 

Roger

 

Edited August 28, 2020 by RogerH

[harlequin]

Hi Rodger

DOT5 (Silicon) aerates but does not remove paint

It may not remove paint but it is almost impossible to repaint an area that has been contaminated by DOT5

As for your questions I am not an expert by any means just a user of DOT5 for a long time so my 2d worth

Q1 No just flush with meths

Q2 No not necessary assuming quality of seals is as original 

Q3 Didnt happen when I changed over to DOT5

Q4 see Q2

The important thing with DOT5 is to have a bottle in the boot on a long trip because about the only place to get it on the road is a Harley dealer 

Tin hat is now firmly in place 

George

 

[Z320]

Hi Roger,

please allow me to add 2 question to your question Q1:

DOT2/3/5.1 does not mix with DOT5! OK!

DOT5 has the lower density and swims on the other DOTs, down in the calpiers below the bleeding nipples there is the DOT 3/4/5.1 how often you ever flash it?

So what is really the problem to use both in the same system (if they don't mix)?

Ciao, Marco

 

[ctc77965o]

Talking facts:

Silicone is a clear liquid or flexible rubber at room temperature.

Silicon is a crystalline metallic-looking solid.

 

Edited August 28, 2020 by ctc77965o

[ctc77965o]

Q3 & Q4: a brake fluid is a mixture of a main base component (glycol,silicone) with different chemical additives designed by its manufacturer to provide a balance of properties. 

One property required is that of controlled swelling of rubber components.. although it may seem counter-intuitive, a little swelling is good to improve sealing (some may recall early Synthetic motor oils lacked swelling agents and caused rubber seals to shrink and fail). Brake fluids contain swelling additives and lubrication additives, among others.

If you mix DOT5 and DOT3/4/5.1 then, although they are "immiscible", there will be a partitioning of the additives... They will move from one of the fluids to the other (this is a basic chemical principle, explained below)...so even though the bulk fluids haven't mixed, their additives have concentrated in one fluid and depleted in the other. This can create a situation where the fluid that has lost additives has lost lubricity, or shrinks seals by extracting swelling agents from the rubber (worst case causing seal to fail). Conversely, the fluid that has gained the additives may now have too much or the wrong additives and it's performance also degraded.

This is the main reason why mixing of DOT5 with DOT3/4/5.1 should be avoided.

US Military study of rubber compatibility of brake fluids:

https://apps.dtic.mil/dtic/tr/fulltext/u2/a082376.pdf

Basics of partitioning:

https://chem.libretexts.org/Bookshelves/Ancillary_Materials/Demos_Techniques_and_Experiments/General_Lab_Techniques/Liquid-Liquid_Extraction/Solvent_Partitioning_(Liquid_-_Liquid_Extraction)

Edited August 28, 2020 by ctc77965o

[ed_h]

That Military study is one of the most informative things I've seen on the topic of mixing fluid types, and is worth 10,000 anecdotes or regurgitations from the Internet echo chamber.  It seems pretty definitive to me: Mixing fluid types is a bad idea, but trace amounts of one in the other may not be a crisis.

Another more recent source of good info is the DOT spec itself.  Though it is silent on mixing fluid types, it lists the rubber materials that any DOT brake fluid must be compatible with.  From the spec, in the "Definitions" section:

"Brake fluid means a liquid designed for use in a motor vehicle hydraulic brake system in which it will contact elastomeric components made of styrene and butadiene rubber (SBR), ethylene and propylene rubber (EPR), polychloroprene (CR) brake hose inner tube stock or natural rubber (NR)."

Ed

 

[ianc]

Thanks very much for bringing the study by the US Military to our attention, Dave.  As Ed remarks, worth so much more than all the stories which are bandied about, and very reassuring for those who use Silicone Brake Fluid (for 27 years in my TR4).

I noted in particular Section 2.3 h(1) & (2) - both very reassuring and support for ensuring that Silicone is used in a system which is completely free of conventional brake fluid, a condition best achieved by thorough flushing (unless all new pipes are replaced with Kunifer) and by replacing all the parts containing rubber.

Ian Cornish

[RogerH]

HI Folks,

the devil is back -

Thanks for your comments.

Dave's US Army report (that must have taken some hunting down) looks interesting but when you read deeper into it, it is full of poorly worded information.

The report is to understand the effects of SBF on the various rubber components and not to understand the mixing of SBF and PEG - although they do look into contaminates in general.

They mix up phrases that contain 'conventional' fluid with engine oil, hydraulic oil (whatever that is) and shock absorber oil - so what is going where.

Why do they not stick by naming the base material - mineral, synthetic PEG etc

 

The report was written in the late 70's and with bespoke tweeking by the manufacturers they state that the Silicone fluid has equal or more lubricity than PEG

That is not actually an accolade as the PEG has more than enough of lubricity and has also improved over the years with better temp etc.

 

Para 1.4 discounts swelling issues but then complains about possible swelling issues further in the report.

Para 1.5 is happy to flush the system but no mention is made of changing ANY rubber components

The lists of test results give a multitude of numbers but what do they actually mean !!!

 

To me the report is probably good for the lab but does not communicate whether SBF and PEG can be mixed and what should you do to the

brake system to ensure it works well.

 

It has been commented previously on this forum as to why the motor industry in general does not investigate this matter.

The general thought was that they have no interest in SBF as it aerates to easily and would harm the ABS.

 

So my four questions still stand.

 

Roger

 

[ctc77965o]

Lubricity of DOT3/4 never met Rolls Royce's expectations, they used precision cam-driven pumps to pressurise their brake fluid to 3000psi in the Shadow/Spirit/Spur cars. A special Castrol DOT3 based fluid, Castrol R363 was formulated specificly for RR cars..it's got about 10% of propoxylated castor oil added to ensure  those precision pumps don't seize. 

Edited August 28, 2020 by ctc77965o

[RogerH]

Hi Dave,

there is always an exception . Didn't RR also use mineral oil for a similar reason as did Citroen

Roger

[AndyR100]

seems pretty straight-forward to me.... if you choose to mix (not sure anyone would make an active choice to go 50:50) then be prepared for some different behaviour from the rubber components exposed. - in practice you are unlikely to notice any catastrophic effects because they'll take a while to happen. 

flushing to the n'th degree is probably going over the top - if you can purge it all that would be good practice but the sky won't fall in if you've left a bit of the previous inhabitant in the system.

If you have a major loss of fluid and need to fill to complete a journey, having a mix of fluid will be better than no fluid.

hard-hat on ....... Andy

[stillp]

When I was a development engineer working on some plastic optical components, we had a target customer whose spec included resistance to petrol, isopropanol and to MEK. After a lot of research we found a coating that would withstand each of these, made some samples, sent them to the customer... who failed them. Our components would resist any of those chemicals individually, but after exposure to one (I forget which, this was in the eighties), another would attack the plastic. The local university found that the first solvent had had some effect on the coating that made it liable to attack by another.I'd be nervous about using silicone fluid in a system that had previously used glycol unless the rubber seals were changed, in case something similar occurred.

My 4A has been on silicone since I finished the rebuild in 1989, with no problems in the braking system, which I rebuilt with all new seals, fixed pipes and flexible hoses. The clutch fluid, again silicone with all new seals and pipes, went black when I first started using it - presumably due to fine rubber dust from the seals, which has now settled at the bottom of the m/c (and probably elsewhere in the system but it isn't causing any problem).

I spilt some silicone fluid on a rear wing many years ago, but didn't have any problems when I repainted it some years later, just used a decent wax & silicone remover.

Pete

[ctc77965o]

19 hours ago, RogerH said:

Dave's US Army report,..... is probably good for the lab but does not communicate whether SBF and PEG can be mixed and what should you do to the brake system.

If you mix 50:50 SBF and PEG you get Devil's Advocaat

[RogerH]

Brilliant