Flushing for Dot 3 to Dot 5, what to use?

[dodj]

Liquids aren't compressible, so if ones brake pedal is soft using DOT5, that is because it is not fully bled. 

Yes they are. Liquid compressibility is quite varied. Just not as much as gasses.

EDIT*** But I will also agree with you on the brakes. If the pedal is soft, probably need to bleed. ***

[tparker]

I have always been told fluid is not comprisable and a simple google search shows mixed answers. However, looking into the physics of it, fluids are comprisable. But to what degree? Can you exert enough pressure to compress it? The consensus is it is very little and it probably isn't a factor in brake fluid. There are equations on line if anyone wants to chunk through it.

However, heat does expand liquid (which is directly related to expansion) and this might be more of an issue when brakes get hot. One reason we have that rubber on the master cylinder, so it can expand and contract as the brake fluid volume changes.

[JH27N0B]

Fluids are compressible, but only to a very small degree.  For example, gallon of water weighs 8.3 lbs at sea level, but if you went to the bottom of the ocean at a deep spot that gallon of water might weigh 8.5 lbs or something.  Calculating how much a gallon of water weighs at a depth of 10,000 feet would be an interesting calculation for a science class, or if someone wants to dig around on the internet you could figure out how to calculate it.
But as far as compressing any liquid in a brake system, you'd have to apply enough force on the brake pedal that you'd no doubt blow out the brake hoses and seals before you'd be able to compress any brake fluid enough to feel a difference.  And you'd have to have leg strength such that your thighs would be the diameter of a Sequoia tree!
I was an engineer in the brake cylinder group for a well known automotive and light truck brake parts company for 14 years and I never heard anything at my work about our products working differently with Dot 3 and Dot 5 brake fluid.  In fact, we sold parts to the US Military and USPS that used DOT 5 as factory fill.  The only reason the parts numbers were different from parts for commercial equivalents for these products was that they had filler caps that said "USE ONLY DOT 5 BRAKE FLUID" instead of "USE ONLY DOT 3 BRAKE FLUID"
https://www.bigmikesmotorpool.com/products/copy-of-service-data-plate-for-m35a2-10872183#:~:text=MIL%20SPEC%20DOT%205%20SILICONE%20BRAKE%20FLUID%20USED,FOR%20YOUR%20TRUCK%2C%20BUT%20THE%20STUFF%20YOU%20WANT%21%21
I remember filling my virgin brake system of my T/A with DOT 5 several years ago when I was restoring it, and it took forever to bleed it.  I prebled the master cylinder, gravity bled the system, then had a friend over and we must have spent over an hour and ran a ton of expensive DOT 5 through the system before we finally got a nice firm pedal.  Dot 5 can be a pain to bleed I know firsthand, but trying to claim you can't get a firm pedal with it when fully bled is just "fake news". 
As far as the "rubber in the master cylinder", are you referring to the filler cap diaphragm?  The reason it expands is to keep the amount of air in the reservoir to little to none, as there is water in the air that you don't want to be absorbed by the brake fluid (Dot 3 and 4).  As your disc brakes wear, the amount of fluid in the master cylinder reservoir decreases, and the diaphragm expands to keep the amount of air above the fluid to a minimum.

[torredcuda]

Fluids, even hydrolic oils are compressible but only to a very small degree. "Contrary to popular belief, liquids are compressible. For hydraulic oil, a rule of thumb is that it will compress 0.5% for every 1000 psi that is exerted. However, for most applications this side of a flight simulator, we can consider it incompressible"

[moparroy]

Have used DOT 5  silicone in all my cars (and Gold wing) for 30 years. Started for the same reasons - paint and hydrophobic (after replacing seized calipers on a stored car - no problems since).
To switch I just flushed through with DOT 5. Then again a year later just to check clarity. In one case I still found traces of DOT 3 years later when I rebuilt the wheel cylinders but no impact on performance. (pistons in wheel cylinders are outside the seals and the pistons got grimy - seals were still fine)
I have read recently about concerns with ABS but I have it in 3 ABS vehicles too - two of them daily drivers and no issues. The one car I hesitated on was my 2010 Challenger since the reservoir is shared with clutch and no way to bleed out the clutch - but I did it anyway a couple years ago because it sits a lot. So more than a trace of mixing there. Had a real life panic stop situation in that car since then - brakes exceeded my expectations - I really thought I was done - still can't believe it when I think back.

[Filthy Filbert]

Yup, just to reinforce... in THEORY, fluids are considered  incompressible because in reality, they compress very little. 

That's why testing a pressure vessel to burst is done with liquid instead of air.   Even as low as 700 psi, a tank full of air rupturing is like a bomb exploding; the massive decompression and rush of air can rip it to shrapnel and send it flying...and the sound!!  It will BOOM like a bomb.  Same tank full of water at 700 psi, and it makes a tiny 'pop' sound and nothing goes flying.

In a pressure system like brakes, you will have more "master cylinder" travel due to the lines flexing/expanding under pressure than you will from the fluid compressing.

[tparker]

OK, we got the compressibility figured out. But what's the deal with switching because brake fluid destroying paint? The break system really isn't by any body paint except for the master cylinder and the lines in the engine bay. Is it because we might spill it when putting it in or a catastrophic failure? Honestly, I am more concerned about flammability of brake fluid. I had a leak in the rear flexible brake hose that would spray a stream on the muffler and then catch fire.

It looks like Glycol is the culprit. Isn't this the same stuff found in Antifreeze? A quick search says anti freeze is bad for old paint but not new paint. I'm assuming eurothane paint isn't affected. Do people avoid Antifreeze in their systems? I've never seen a catastrophic break issue but I have seen radiator hoses go and radiator caps removed under pressure all resulting in fluid EVERYWHERE.

Just currious

[GoodysGotaCuda]

Guy comes in asking about how to flush it and ends up with 20 posts on fluid dynamics!

I don't believe you'll find a textbook answer that says you can fully clean out the old, but I would use alcohol.

When bleeding the DOT5, let your fresh "bled" fluid settle on the bench for a while, if you plan on putting it back through the system to continue bleeding.


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[Brads70]

Guy comes in asking about how to flush it and ends up with 20 posts on fluid dynamics!

I don't believe you'll find a textbook answer that says you can fully clean out the old, but I would use alcohol.

When bleeding the DOT5, let your fresh "bled" fluid settle on the bench for a while, if you plan on putting it back through the system to continue bleeding.


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[moparroy]

OK, we got the compressibility figured out. But what's the deal with switching because brake fluid destroying paint? The break system really isn't by any body paint except for the master cylinder and the lines in the engine bay. Is it because we might spill it when putting it in or a catastrophic failure? Honestly, I am more concerned about flammability of brake fluid. I had a leak in the rear flexible brake hose that would spray a stream on the muffler and then catch fire.

For me it was more about changing to a hydrophobic fluid. Early years in winter storing my Daytona ('85) I had issues with calipers seizing while it was stored over winter. Now I was probably not changing brake fluid often enough - but the rust on the pistons I blamed on moisture absorbed by the hygroscopic Dot 3. I happened one day to be looking over brake fluid selection and picked up a DOT 5 bottle partly because it had a longer life rating and reading the label thought this would solve my problem and indeed it did. That fluid was rated 5 years and I still did not change it that often but I have never had another issue with calipers seizing of any other corrosion issues in the brake system. I don't subscribe to the theory that hydrophobic fluid will cause pooling in the system - rather my experience is that because it does not like water, the moisture never enters the system at points where DOT 3 might draw it into the system due to its nature to like to mix with water. The paint thing was a side benefit. I tend to keep my daily drivers a long time too - so this makes one less maintenance thing to worry about or at least with longer intervals.

[dodj]

Guy comes in asking about how to flush it and ends up with 20 posts on fluid dynamics!

I think it was that post #3...

[JH27N0B]

In the end it's all good, because brake fluid types is an interesting subject to car enthusiasts and restorers, and even though we maybe went off on some tangents to the OPs original question, hopefully there is a lot of good information that's come out in this thread about DOT5 which members will use as reference for years to come.
On a related amusing note, I first used DOT 5 in the early 90s when I replaced a lot of components like calipers and master cylinder on my car and thought while I'm at it, I'll change to DOT 5 brake fluid.
At the time I was renting a room in my house to a friend of mine who owned a new Goldwing motorcycle which he parked in the garage by my Challenger.  He saw me bleeding my brakes near his bike and said he didn't like me doing that as he was afraid some brake fluid would get on his bike and damage the paint. I said it's DOT 5, silicon, it won't damage paint.  He said "I still don't like you bleeding brakes by my bike!". I repeated, "it's silicon, it doesn't damage paint!". Then I proceeded to pour a little brake fluid on my fender, then got a cloth and polished it, to show it didn't damage paint.  He was the type who always had to get in the last word, so he says "I still don't like you bleeding brakes by my bike" again, then turned and walked away. Which spared me from having to tell him if he didn't like it, he didn't have to park his bike in my garage. 

[torredcuda]

Guy comes in asking about how to flush it and ends up with 20 posts on fluid dynamics!

I don't believe you'll find a textbook answer that says you can fully clean out the old, but I would use alcohol.

When bleeding the DOT5, let your fresh "bled" fluid settle on the bench for a while, if you plan on putting it back through the system to continue bleeding.


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Ya  but good discussion. As I said before I use Dot 5 for two reasons - it doesn`t harm paint, which I know is only an issue when filling or beeding and it doesn`t absorb water which is good since our old cars usually sit for long periods of time (winter).