Testing Brake Fluid

That would vary wildly depending on the multimeter. Completely unreliable. Google brake-chek for a german optical tester.

"ED" wrote in news:ittehg$a0s$ snipped-for-privacy@dont-email.me:

Trying to avoid getting the fluid changed?

in theory it could work, but it depends on the electrode metal and the type of metal the cylinder is made of. cast iron has a different electrode potential to aluminum, which has a different potential to stainless, which has a different potential to chrome plated brass, which has a different potential to only partially chrome plated brass, etc. so, unless you know the potentials for your probes and your system respectively, or you use a specific known probe calibrated for the system you're testing, you're completely in the dark.

Problem is that this is very dependant on the electrodes being used. If you had a couple standard electrodes, though, you could do it accurately.

--scott

I dont think even that would help this method, Scott.

The most accurate method I know of for water in polyols is Karl Fischer titration....and that is beyond the cost limitations and technology of most people who want a value for water in polyols.

There are some test strips:

Discussion:

there aren't many modern vehicles with brass brake componentry - "copper" is just something to get the ignorati worried about so they pay for more diagnostics and the service industry concerned that they "don't get left behind" and buy new equipment.

as for test strips, they're ancient history.

It's true that there's other stuff that will increase conductivity, but I think everything that will possibly increase conductivity is going to be something you probably don't want in your brake lines.

But, unless you have calibrated electrodes and a calibrated volume, you can't get resistivity from resistance.

That seems severe overkill for the application, and definitely not an easy kitchen job.

Frankly, it seems easier to change it than to test it, although I realize some newer vehicles with ABS systems can make it more difficult to change.

--scott

ain't that the truth. while i'll take testing over superstition every time, brake fluid, with its attendant seal swelling/degradation problems, and known absorption profile, is simply not worth it - just change every other year. testing may be useful for shops where they have a new customer and an unknown service profile, but for an owner that knows their vehicle's history, it's pointless.

that may have been the case when they first came out, but these days, it's really not a problem.

This is true. Conductivity or resistivity measurements will respond to various types of contaminants in the brake fluid, to temperature, etc. But maybe you dont really need a very accurate measurement.

I think it would be cheaper and better just to flush out the old stuff and replace every couple of years.

I still have that #@& Reatta that has the very special braking system. If you dont flush that one every couple of years, you get to buy a new or rebuilt master cylinder, which - last I heard - cost the better part of a couple of grand.

I like to peer into the reservoir and change it if it looks nasty. Your way seems a lot easier though. :-)

If you use the chassis of the car, that is very problematic.

If you had two parallel electrodes of the identical metal type and a fix le ngth exposed to the brake fluid, then readings would be meaningful, if not precise.

Let's say you made such a test probe and calibrated it with fresh brake flu id that is mixed with various percentages of distilled water. Now you could put this probe into a random brake fluid reservoir and get a repeatable re sult. The resistance reading would depend on the electrode separation as we ll as the uninsulated length.

It would probably provide an inaccurate reading due to soluble metals, salt s and minerals contaminating the brake fluid. However that would only cause errors on the side of caution. If it gave a high enough resistance reading , that would indicate the reservoir was dry.

The next issue to worry about is: how fast does moisture migrate from wheel cylinders back to the master cylinder reservoir? I don't know. However, yo u could flush a small quantity from each bleeder valve and test that in som e very small sample container (or directly in-line with the bleeder hose). Flush until the reading is good enough. This would be an excellent way to c onserve on brake fluid, if that was a significant business expense.

If you use the battery, chassis of the car, or master brake cylinder as an electrode, that is very problematic. You have voltages created by electroly sis with the master brake cylinder metal alloy. You have variations in the current path taken between the electrode and the reservoir. Finally, variat ions in the surface area of the conductor probe immersed in the reservoir.

If you had two identical parallel electrodes of the same metal exposed to t he brake fluid, then readings would be precise and repeatable,if not meanin gful. The resistance reading would depend only on the electrode separation as well as the uninsulated length.

Let's say you made such a test probe and calibrated it with fresh brake flu id that is mixed with various percentages of distilled water (0,1,2,3,4,&5% ).

Now you could put this calibrated probe into a random brake fluid reservoir and get a result everyone could agree on.

It would probably provide an inaccurate reading due to soluble metals, salt s and minerals contaminating the brake fluid. However that would only cause errors on the side of caution. If it gave a high enough resistance reading , that would indicate the reservoir was adequately dry.

The next issue to worry about is: how fast does moisture migrate from wheel cylinders back to the master cylinder reservoir? I don't know. However, yo u could flush a small quantity from each bleeder valve and test that in som e very small sample container directly in-line with the bleeder hose. Flush until the reading is good enough. This would be an excellent way to conser ve on brake fluid, if that was a significant business expense.

For readers that wonder why: Water content is very bad for brakes. A hot brake cylinder will exceed the boiling point, turning water into steam.

Brake fluid is designed to dissolve water and draw it away from the brake c ylinders. The master brake cylinder is connected to the reservoir when the pedal is up, so this moisture is also diluted into that large volume as wel l.

If you use the battery, chassis of the car, or master brake cylinder as an electrode, that is very problematic. You have voltages created by electroly sis with the master brake cylinder metal alloy. You have variations in the current path taken between the electrode and the reservoir. Finally, variat ions in the surface area of the conductor probe immersed in the reservoir.

If you had two identical parallel electrodes of the same metal exposed to t he brake fluid, then readings would be precise and repeatable,if not meanin gful. The resistance reading would depend only on the electrode separation as well as the uninsulated length.

Let's say you made such a test probe and calibrated it with fresh brake flu id that is mixed with various percentages of distilled water (0,1,2,3,4,&5% ).

Now you could put this calibrated probe into a random brake fluid reservoir and get a result everyone could agree on.

It would probably provide an inaccurate reading due to soluble metals, salt s and minerals contaminating the brake fluid. However that would only cause errors on the side of caution. If it gave a high enough resistance reading , that would indicate the reservoir was adequately dry.

The next issue to worry about is: how fast does moisture migrate from wheel cylinders back to the master cylinder reservoir? I don't know. However, yo u could flush a small quantity from each bleeder valve and test that in som e very small sample container directly in-line with the bleeder hose. Flush until the reading is good enough. This would be an excellent way to conser ve on brake fluid, if that was a significant business expense.

For readers that wonder why: Water content is very bad for brakes. A hot brake cylinder will exceed the boiling point, turning water into steam.

Brake fluid is designed to dissolve water and draw it away from the brake c ylinders. The master brake cylinder is connected to the reservoir when the pedal is up, so this moisture is also diluted into that large volume as wel l.

Are you looking for something like this ?

The problem with testing brake fluid is then you're exposing it to the atmosphere. It'll soak up all the moisture available. Better to do what I do. Leave it in peace.