Testing Brake Fluid with a Voltmeter

[barryh]

To clarify, the 7 month old fluid in my car measured 5mV (0.005V).

I tested the darker 18 month old fluid in my motorcycle today and the reading was 25mV (0.025V.)

The reading fluctuated, starting high at 50mV and then falling slowly to 25mV. I am using a digital meter set on the 200mV range so the input impedance should be reasonably high but it still seems to have an impact on the reading. The depth of insertion of the probe also caused the reading to fluctuate which I suppose is what you would expect.

It seems I have a long way to go before reaching 0.3 V and will check again when the fluid is 2 years old. I'd have to say I'm not yet convinced that it's a reliable test particularly the 0.3 volt threshold.

 

[jrvn]

Originally Posted By: barryh
To clarify, the 7 month old fluid in my car measured 5mV (0.005V).

I tested the darker 18 month old fluid in my motorcycle today and the reading was 25mV (0.025V.)

The reading fluctuated, starting high at 50mV and then falling slowly to 25mV. I am using a digital meter set on the 200mV range so the input impedance should be reasonably high but it still seems to have an impact on the reading. The depth of insertion of the probe also caused the reading to fluctuate which I suppose is what you would expect.

It seems I have a long way to go before reaching 0.3 V and will check again when the fluid is 2 years old. I'd have to say I'm not yet convinced that it's a reliable test particularly the 0.3 volt threshold.


my digital volt/ohm meter was set at 20 DC Volts the screen shows the decimals like this 0.00, it went rather quickly to .35 volts on a few occasions that I checked it, so I was consistently getting the same reading.

 

[jrvn]

heres another good link on how to check for contaminated brake fluid with a voltmeter https://media.supercheapauto.com.au/sca/images/articles/Repl_Brake_Fluid.pdf

 

[JoeJ]

Interesting thread for sure! I'm not even going to test my brake fluid. I know it's old and time for change. I'm going to Harbor Freight and get the 1 person vacuum brake bleeder kit. Beats the heck out of an afternoon yelling "Push & Hold...now let it up..." to my wife.

 

[barryh]

If you get chance to test the brake fluid we will have another data point and if it's that bad we might discover if the 0.3 volts is possible.

 

[GR_Winchester]

I know that this is an old thread but I'd like to note that for accuracy when testing one's coolant or brake fluid with a digital meter, it's important to test a sample that has been removed from the reservoir rather than checking the conductivity or resistance within the reservoir/MC. Plenty of conditions could otherwise affect one's readings.

Furthermore I should mention that brake fluid, especially DOT 4/5, are hydroscopic in nature and water and air will begin mixing homogenously with one's fluid the moment one cracks the bottle of whatever aged fluid they have on that shelf in the garage. Brake fluid is also prone to photodegradation, and those bottles probably don't filter non-visible light, or perhaps even visible light, in entirety.

I would be wary of using an unopened bottle of brake fluid that's been sitting exposed for more than four to eight months (varied by temperature and light exposure), given that it has already spent who knows how long in a vat, a warehouse, in transportation, a distribution center, and in a store. Even if you keep it in a wine celler; after the two year mark, go invest in peace of mind and spend $3-6 on a quart of new fluid.

 

[Patrick0525]

Originally Posted by jrvn
to end my saga I took my truck to the Toyota dealer that has a BG pressure/power bleeding machine to purge all the old brake fluid out, and the peddle feels very good now(how it should feel) they also checked the old fluid for moisture with test strips and found some moisture but they didn't seem to think it was too much my voltage test says the fluid was no good. before they did any work,the tech also test drove the truck before hand and thought the brakes were OK

but I can tell you with certainty that they are very much improved after they BG flushed the system total cost was $128 and worth every penny of it IMO.



My dealer would have charged $120 for my Toyota brake fluid flush but I supplied my own brake fluid Wagner DoT 3 (2qt -$15) and they charged $85 for labor.