...is there a DIY way to tell if what's currently in the car is the real DOT5 vs all the others?

I ask because way back when (so about 7-8 yrs. ago) when I finally finished restoring my coupe I was excited about using DOT5 primarily due to it's paint friendliness. It did help that DOT5 wouldn't absord water as well, and this being a non-ABS application I thought it made sense to go that way. But that was some years ago and while my build sheet says I put DOT5 in there, I honestly can't tell for sure (second quessing myself here given the years that have passed).

Well, I was looking at the MC the other day, the fluid in there seems sort of brownish to start off with (and I thought the DOT5 stuff was purple - I used Valvoline), and some of it dripped down on my re-finished power steering coupler. So I quickly went to wipe it clean (being used to the regular brake fluid impact on paint), and while it wiped nicely it looked like it dulled the finish...and that's when I started to think: "....Ummm...maybe that wasn't DOT5 I stuck in there to start off with????..."!!!

Sooo...that begs the question: is there a DIY test I can do at home to positively determine if this is a DOT5 fluid that's in there right now?

I thought of actually taking some of it out and putting on a spare finished part (car paint) to see what it'll do...but is that good enough of a test?

Thanks!

Iirc, dot5 doesn't mix with dot3 or 4. Take a bit of known dot3 and put it in a clear cup. Ad a sample of what is in your car.

If you used dot5 it will form a separate layer in the cup. Just like water and oil.

DOT 5 is distinctly purple when new but can fade with time. The others start as yellow but often end up much darker.

Here is a good motorcycle guide on brake fluid that does a good job of answering your question:

www.youtube.com/watch?v=FhBde4QtxPo

Take a piece of scrap metal, paint it, put some of your mystery fluid on it and see if the paint comes off.

if it has been 7 or 8 years, it is time to replace it anyway.
I wouldn't worry about what it was, flush and put in new dot 5.

Mine was purple & turned dirty lookin after a year

Bleed some out and test it on some paint. Maybe once it’s in a jar you can see if it’s purple tinted?

If you want to test the fluids with the least waste
use a scale and a small measuring container
to calculate the density.

I would use the small cheap Walmart “baby medicine oral syringe” and a kitchen scale that can read out in 1 gram increments.

Walmart also sells a small syringe with a special tip called a “ear wax flusher”.

Walmart also sells a much larger syringe they label the “2 cycle fuel mixing” tool in the automotive section that would work with kitchen scales that only read out in 5 gram increments.

These syringes can be used to “CC cylinder heads” and to push lubricants to hard to reach spots through small tubing, like the various diameter tubing you can salvage from spray cans and “suction tubes” from pump bottles.

Pull some, and mix it with water in a small glass jar, is what I would do.

This works, dot5 will separate from water.

Thanks you guys, excellent ideas all, especially the mixture stuff: non invasive, non destructive!

re: total bleed and replace with something like DOT 5.1, I didn't think this was a "go" since I've always read that literally ALL components have to be replaced, so that would include any soft parts, drum brake cylinder rubber caps, disc seals, etc...is that not the case?

Some may not agree, but I would stay with the DOT 5.

Who would make a fluid such that, if mis-used or didn't follow direction, could result in brake failure.
Who would do that? If this was true, you'd have to sign a legal wavier in front of a notary public, and have it witnessed by two certified lawyers, one of your own, and one from the manufacturer, all before you could buy an ounce.

This is one of those rumors that will never ever go away.

For what it's worth, my late friend Nick S, a PHD chemist, gave me some DOT 5 fluid in the later 1980's. I used it to fully rebuild the drum brake system in my 64 D100 in about .1988.
The wheel cylinders started failing in about 2021 or so, and I had to replace them.
The fluid owed me nothing, as they say. I use it in everything old school now.
Mark

OK, so I've got some results, take a look at the attached pics.

Basically the old brake fluid is the clumpy brown stuff, the jar has water in it otherwise.

Upon dropping the brake fluid into the jar the two completely intermixed, there was NO separation. The results you see here is what I found the following morning, so about 12 hrs. later as the mixed brake fluid in that water medium settled out.

I will test against a NEW DOT3 fluid next...

Also, sorry about delay in responding...my motor is finally back in the coupe, all things have been re-connected so TODAY is the big day, we re-start unless we get rained out! lol (the plan was for yesterday but it pretty much rained the whole day and I have to get the car out of the garage for that event)

looks like DOT 5 to me.
I think I would to see some silicone implants to be sure. Maybe some hands on experimentation. Sorry just saying what I was thinking.

Here is how it was explained already by B1MAXX. When you see the results (not mixing with water by DOT 5 silicone base) you will see one of the bad points or reasons some say not to use it unless you change it frequently. Water not mixing with DOT 5 means the water sits in some points of the entire system where it comes in direct contact with surfaces and corrodes or rusts (steel) lines from the inside out. Not that this happens overnight but it does happen over a period of time. Just passing this on from M/C industry experience and especially ABS complicated brake systems which most classic vehicles don't have.