Got a brake fluid moisture tester

[eljefino]

These things are two bucks on ebay! Here's my review.

-- It does something resembling working, based on what I know about my vehicles. It has LEDs in green for zero percent, amber for 1 & 2 percent, and red for 3 and 4+ percent, labelled "bad."

-- 91 F150, did rear brake lines, bled, but didn't suck the MC dry beforehand. Fluid inside is dark. The amber "one-percent" LED flickers.

-- My vat of used brake fluid shows 2%.

-- My 4.5 year old Prius shows 0%.

As for use of the tool, it gives its best reading when you really stab the "fork" half an inch into the fluid. Bring a rag, it gets drippy. The LEDs are not optically shielded from each other within the guts of the thing, so the light transfers from "lit" to "unlit" fairly readily, making it less elegant to read.

Is it calibrated? Who knows, I would say it strays towards false negatives if you're supposed to change fluid every two years. Is it effective? Likely, if it tells you to change it, you really probably should. Is it worth the two bucks? IMO, yes!

I "could" get a calibrated beaker, some new fluid, and mix in some measured amounts of water for a definitive test... haven't yet though.

 

[cptbarkey]

it would be interesting to see how it compares with the test strips. I was glancing at them a few days ago. Thank you for sharing.

 

[benjy]

moisture as well as dirty fluid is surely important + prolly often overlooked!!

 

[barryh]

There is another way to do it for zero cost although it's possibly even less calibrated.

If you set a digital voltmeter to a low dc voltage range and put one probe in your brake fluid reservoir and the other to earth, it will read a voltage created through galvanic action in the brake system. We have already talked about in previous threads. The link below suggest 300mV volts as a limit to indicate fluid that needs to be changed but when I tried it I read only 5mV on nearly new fluid and 35mV on some older fluid. I must try it again tomorrow because the fluid in the car really should be due for a change.

In fact I'm too curious now to wait until tomorrow, I've just been in the garage to do a reading on 2 year old fluid and read 100mV. Even though the fluid is a good colour I'll take that as an indication that it needs doing very soon.

https://www.linkedin.com/pulse/motor-vehicle-brake-fluid-fundamentals-testing-kiril-mucevski
https://bobistheoilguy.com/forums/threads/testing-brake-fluid-with-a-voltmeter.278784/

 

[The Critic]

For $2, that is hard to beat.

With that said, I think too much emphasis is placed on moisture - there are other reasons why brake fluid may need to be changed.

 

[Jetronic]

For $2, that is hard to beat.

With that said, I think too much emphasis is placed on moisture - there are other reasons why brake fluid may need to be changed.

If you wonder if it might need replacing, it surely will.....

 

[stangguy]

Here is my deal. I have had many cars over my 65 years of driving, but have never kept many more than a few years. Most were quite old with lots of miles when I bought them, but one, my 65 Mustang in about 2000 and sold in 2023, and I never did change the brake fluid in any of them. And never any ptoblems

 

[MattPersman]

For $2, that is hard to beat.

With that said, I think too much emphasis is placed on moisture - there are other reasons why brake fluid may need to be changed.

A good customer would change all their fluids each visit

 

[4329]

These things are two bucks on ebay! Here's my review.

-- It does something resembling working, based on what I know about my vehicles. It has LEDs in green for zero percent, amber for 1 & 2 percent, and red for 3 and 4+ percent, labelled "bad."

-- 91 F150, did rear brake lines, bled, but didn't suck the MC dry beforehand. Fluid inside is dark. The amber "one-percent" LED flickers.

-- My vat of used brake fluid shows 2%.

-- My 4.5 year old Prius shows 0%.

As for use of the tool, it gives its best reading when you really stab the "fork" half an inch into the fluid. Bring a rag, it gets drippy. The LEDs are not optically shielded from each other within the guts of the thing, so the light transfers from "lit" to "unlit" fairly readily, making it less elegant to read.

Is it calibrated? Who knows, I would say it strays towards false negatives if you're supposed to change fluid every two years. Is it effective? Likely, if it tells you to change it, you really probably should. Is it worth the two bucks? IMO, yes!

I "could" get a calibrated beaker, some new fluid, and mix in some measured amounts of water for a definitive test... haven't yet though.

View attachment 202656

View attachment 202661View attachment 202662

A boiling point tester is the only accurate way to test brake fluid performance. Refer to brake and fluid manufactures.

 

[Kantaro]

If you have new brake fluid to spare, you could validate the accuracy of your tester:



I do wonder if the moisture content in the reservoir is representative of the whole system though?

 

[totegoat]

Paid $3.37 for mine, works as advertised.

 

[4329]

If you have new brake fluid to spare, you could validate the accuracy of your tester:



I do wonder if the moisture content in the reservoir is representative of the whole system though?

No

 

[barryh]

So I checked the brake fluid voltage reading on my motorcycle this morning The fluid was a similar colour to that in my car i.e. only slightly darkened from new and the car had tested at 100 mV. The bikes fluid read 135 mV.

I changed the fluid in the bike and it dropped to 5 mV. I'm going to own up now and say that the fluid I used was what I had on hand which was the same 1 litre tin of Ate SL6 which had been on the shelf since the last time I did these measurements when we discussed it back in Nov 2017. That makes it over 6 years old. Now I'm not suggesting anyone use such old fluid but 5mV is what I read 6 years ago using new fluid from the same tin. That suggests it can't have absorbed much moisture in storage. I will check it again in the summer to see if the reading has changed and buy a new tin of SL6.

 

[willbur]

Not a surprise. Its the metals leaching from brake components that increase the mv. Unused 100 yo brake fluid would probably read the same LOL. Moisture content is way over rated for brake degradation-its the leached metals that do the damage IMO.

Moisture in brake fluid is probably the third most discussed topic here behind AZ on spark plug/lug nut threads and "what oil should I use?".

 

[72C10]

I bought a brake fluid tester and it kept failing brand new bottles of brake fluid. Now maybe there was high water % in those new bottles I tested but I kind of doubt it.

So now I just change the fluid every 2 years in my vehicles.

 

[CarDoc66]

I have the same tester and have found “cheaper” brands of fluid show some moisture content right out of the bottle. I thought it was the tester itself but I checked a new bottle of genuine BMW fluid and it did not register any moisture.

 

[72C10]

Prestone DOT 3 brake fluid, new in the bottle was what kept failing for me.

I had no way of proving that a $10 tester was accurate vs the brand new bottle.

 

[LongTime_Lurker]

I have the same tester and have found “cheaper” brands of fluid show some moisture content right out of the bottle. I thought it was the tester itself but I checked a new bottle of genuine BMW fluid and it did not register any moisture.

Is the cheaper brake fluid Dot 3 and the BMW fluid Dot 4? Some of these testers arrive calibrated for one or the other.

 

[CarDoc66]

Yes, the BMW stuff was DOT4 or 5 I can’t remember precisely but it wasn’t DOT3. It’s also a bit of a golden color and seemed to have a thicker consistency to it.

 

[eljefino]

So I checked the brake fluid voltage reading on my motorcycle this morning The fluid was a similar colour to that in my car i.e. only slightly darkened from new and the car had tested at 100 mV. The bikes fluid read 135 mV.

I changed the fluid in the bike and it dropped to 5 mV. I'm going to own up now and say that the fluid I used was what I had on hand which was the same 1 litre tin of Ate SL6 which had been on the shelf since the last time I did these measurements when we discussed it back in Nov 2017. That makes it over 6 years old. Now I'm not suggesting anyone use such old fluid but 5mV is what I read 6 years ago using new fluid from the same tin. That suggests it can't have absorbed much moisture in storage. I will check it again in the summer to see if the reading has changed and buy a new tin of SL6.

This is actual empirical evidence vs conjecture, however, and I thank you for the contribution. I as well use several-year-old fluid from an open container with good luck. I surmise that a mere cubic centimeter of fluid, stagnant in the dead-end of a system in a wheel cylinder, degrades substantially and causes havoc. It's this, you want out. A complete flush is cheap and should be done once you're in there, because, heck, you're in there, and there's still the question of boiling point.