Brake fluid question

[Joe Johnston]

Is there a way to tell what type of brake fluid is in the system? I know what is in my cars, but for example you just purchased a car and need to top off the reservoir. Any easy way to tell what is in the system?

[simplyconnected]

Sure! DOT-3 is water soluable. DOT-5 is not because it's silicone based. So, if you take a small sample (like an eye dropper) and put a few drops in a glass of water, watch what it does. DOT-5 floats on top. DOT-3 disburses because it's glycol based.

[Joe Johnston]

Awesome!! An answer with an easy way in 4 minutes!

Why didn't I think of that?? I knew DOT 3 absorbed water and DOT 5 didn't! Just never put it together like that.

Thanks

[simplyconnected]

There are those who swear by DOT-5. I have that in my Harley (from the factory). I'm not impressed at all with the results.

Water still gets into the brake lines from condensation, etc. The difference is, it pools in low areas like caliper and rear wheel cylinders. Then it rusts.

OEM's still use DOT-3 because there is nothing better. Please change it every few years because it can only absorb so much water before saturation sets in. The whole system only holds a cup. It's beneficial to crack those bleeder valves before they freeze, too. If your brake fluid is redish in color (open the M/C and look), rust has already started on the INSIDE of your brake lines. New DOT-3 will stop that. - Dave

[Joe Johnston]

I have DOT 5 in the 57 and DOT 3 in the 63. Both were clean last summer, but will be checked again when I get them out of winter storage. Need to change the antifreeze in the Spring too!

[simplyconnected]

You want to change fluid BEFORE it turns red. Water saturation can also cause your brake fluid to boil or freeze. I would be more concerned about freezing than boiling.

In a DOT-5 system, water pools in the lowest parts and it freezes more easily because it never dilutes. It just sits there and rusts or pits. This usually happens inside the caliper bores or wheel cylinder bores. Alcohol dilutes the water and dries it up.

Petroleum products swell all the rubber parts so keep them far away. Brake Cleaner must be alcohol based. I know they are very flammable.

If DOT-3 and DOT-5 are mixed in the brake system, it turns to a spongy muck (unless you flush the system with alcohol right away) that refuses to flow. That means, you're going to replace all your brake hoses and lines, and rebuild all the cylinders.

[GTE427]

Went to flush the brake fluid over the weekend. The vac pulled purple fluid from the master cylinder. Water test confirms DOT 5. I'm holding DOT 3 at the ready, so I stop.

Do you recommend the same flush interval for DOT 5 as DOT 3? Since the water settles to the lowest point, is it critical to ensure a full flush since the fluid high in the system should be pure?

[simplyconnected]

Ken, you were wise to test your brake fluid. The answer to this can be tricky. Bleeder screws are located at the highest points because air bubbles rise. DOT-5 floats on water, so water pools in the lowest places, usually in the bottom of your caliper pistons or in wheel cylinder bottoms.

Everyone has their personal opinion on the benefits of each type of fluid so I won't go into that.

Denatured alcohol mixes with water like Scotch. That's why gasohol never needs dry-gas (remember dry-gas?); the ethanol sucks up any traces of water in your gas tank and the filling station's tanks, too. Saturated gas is stale gas.

I'm glad you didn't mix DOT-5 with DOT-3. That mistake would cost a total teardown of every cylinder and most likely new brake lines. The muck produced in the lines becomes so solid, compressed air won't blow it out.

Since we cannot stop water from entering the system, my personal preference is to use DOT-3, let it absorb water (as it is designed to do) then change it at three year intervals. The only problem with this is, most folks will not do preventive maintenance other than oil changes.

To answer your question directly, you may:
Keep the system filled with DOT-5 and take your chances,

Drain the system, as best you can, flush with alcohol, fill with DOT-3 and flush it again to get the alcohol out after 'working' the brakes to dilute the fluid then flush it again with fresh DOT-3.

DOT-3 is glycol based and not compatible with silicone based DOT-5. Part of the problem is, brake systems do not have a drain at any of the lowest points. - Dave

[GTE427]

Dave,
Thanks for the information. Sounds like there isn't a periodic flushing of Dot 5 required since it doesn't contaminate like Dot 3. However any moisture will settle, requiring the system to be opened at the low point such as wheel cylinder seals.
Does changing back to Dot 5 require replacement of all rubber seals and hoses in addition to the alcohol flush?

[simplyconnected]

Dave,
Thanks for the information. Sounds like there isn't a periodic flushing of Dot 5 required since it doesn't contaminate like Dot 3. However any moisture will settle, requiring the system to be opened at the low point such as wheel cylinder seals.

Each fluid type has its own advantages and disadvantages.
You are correct about flushing DOT-5, there is no advantage in changing to fresh DOT-5 because it simply floats on water.

So, how do we get the water out? There are no drain plugs in caliper cylinders. Rear wheel cylinders are easy, simply pull the parts out and re-do. While in there, you will see rust (and corrosion) at the bottoms of each cast iron bore.

Does changing back to Dot 5 require replacement of all rubber seals and hoses in addition to the alcohol flush?

As far as seal compatibility goes, I believe the seals were either made for DOT-5 or they weren't. I'm not a chemist but I have seen the affects of mixing both fluids; it's really a mess. Seals bloat, cylinders stick with the brakes on, Stop Switches fail (stay on, leak, etc.). I would not attempt a change before doing a complete and thorough flush.

Most mechanics don't, but I rebuild my own calipers. A rebuild kit consists of one seal (looks like a square rubber band) and a dust boot. The cost is $2.01 per caliper at Rockauto.com for RAYBESTOS WK885. The only tools necessary are a block of wood (for safety) a small hammer (for gentle persuasion) and compressed air (to drive the piston out). I use brake fluid with 250 sandpaper to clean the wheel cylinder and caliper bores.

In my opinion, all this is very unnecessary if you simply stay with DOT-3 and change it every few years. It's not hard and you don't need to bleed air out of the system. Besides, it is what your car was designed for. - Dave