DOT 5 Brake Fluid

[jesusgatos]

Ok, another point on bleeding. If you bleed your own brakes, DO NOT use a pressure bleeder unless it is the type with a diaphram or piston between the pressure and the actual fluid. Otherwise air is injected.

How is air going to get injected into the fluid if the pickup is submerged in fluid? I've heard that DOT 5 fluid is particularly susceptible to aeration and foaming if it's shaken-up. I guess maybe I could see that being a problem? Could you please elaborate on this point though, because I just built a pressure-bleeder.

 

[mikeyapproved]

How is air going to get injected into the fluid if the pickup is submerged in fluid? I've heard that DOT 5 fluid is particularly susceptible to aeration and foaming if it's shaken-up. I guess maybe I could see that being a problem? Could you please elaborate on this point though, because I just built a pressure-bleeder.

Very good question... here is the deal:

Having pressurized air in contact with the fluid is the problem. Air is pushing on the fluid, forcing the fluid out of the pressure unit.

This pressurized air forces itself into the fluid. This can happen at a level that is virtually undetectable to the eye as the air bubbles are very small.

So we end up putting air and the moisture trapped inside the air into our brake system! The whole reason we were bleeding the system in the first place is to get rid of air and water!! DRAT!

Anyone who is in the business of doing alot of brake work, stores their fluid in containers which are hooked to a constant vacume, thus keeping the fluid from touching air. Them we come along with our pressure bleeders that actually force 30PSI of air pressure onto and into the fluid!

Well, sorry if I have thrown a wrench into your plans.... Like I mentioned, pressure bleeders can be great tools, but only if the brake fluid and air pressure are kept seperate.

mikey

 

[stumps]

Air and DOT 5 are only a problem if you shake them up. Pressure bleeders don't cause any problems at all. DOT3/4 have problems with air contact because they are naturally hygroscopic.

-Chuck

 

[m16ty]

I (and several others here) have been using pressure bleeders made from an old garden sprayer with good results. I'm not buying into the notion that this type bleeder injects air into the system. Air would have to go from to top of the tank, through the fluid, and into the pickup tube. Just don't see how it could happen.

 

[3rdmdqm]

I have a 68 Deuce which had an 87 rebuild. I am planning on installing a remote brake fluid reservoir as many have mentioned in another recent post. This of course will require adding some additional brake fluid. My question is, how do I know if I currently have DOT3 or DOT5 fluid in the system. I realize I should be able to tell by a purple color for DOT5, however one post mentioned that if the fluid has been in the system for a long time it will lose its purple color. I certainly don't want to make the mistake of mixing fluid types.

 

[stumps]

Put a teaspoon of the fluid into a small jar of water, and then swirl it around a bit. If it is DOT3/4/5.1, it will vanish into the water without a trace. If it is silicone DOT5, it will spread out on the surface and shimmer. You will know it when you see it.

I have a little jar of 50:50 DOT3/DOT5 that I keep around to prove that nothing funny happens, and the only thing funny that happened is the DOT3 slurped all of the purple dye out of the DOT5 the first time I shook it up. After that, they separated so thoroughly that the DOT5 floats 6 inches above the DOT3....just kidding! They are completely non-miscible.

In my experience, 90% of what "experts" keep spreading around about DOT5 is just plain wrong. I have been using it in cars and trucks, some daily drivers, others seldom used, for the last 15years, and all it has done is suspend the brake parts in time. They don't age, they don't wear, they don't form sludge, they don't rust. And the pedal isn't spongy. It's like a brick.

-Chuck

 

[mikeyapproved]

I (and several others here) have been using pressure bleeders made from an old garden sprayer with good results. I'm not buying into the notion that this type bleeder injects air into the system. Air would have to go from to top of the tank, through the fluid, and into the pickup tube. Just don't see how it could happen.

Hi.

Just to clarify, it doesn't push air through the system into the pickup tube. The pressure of the air on top of the fluid can cause the air and fluid to mix down near the molecular level.... When this happens you get very small bubbles mixed with the fluid throughout the fluid. Then this fluid goes through the system with these small air bubbles mixed in.

Since DOT 5 has a much lower viscosity that DOT 3 or DOT 4, the problem is made even worse with DOT 5. The lower viscosity makes it easier for these air molecules to fit in between the brake fluid molecules.... Yup, water doesn't mix with DOT 5, but air mixed in the system will cause less pressure to actually reach the wheels where we require it the most!

Most of us will never notice the difference, as we never use our brakes near the limit.

But if all we need to do to ensure that our brake systems are up to snuff. is to follow the correct procedure, I personally feel it is worth the effort.

mikey

 

[jesusgatos]

Very good question... here is the deal:

Having pressurized air in contact with the fluid is the problem. Air is pushing on the fluid, forcing the fluid out of the pressure unit.

This pressurized air forces itself into the fluid. This can happen at a level that is virtually undetectable to the eye as the air bubbles are very small.

So we end up putting air and the moisture trapped inside the air into our brake system! The whole reason we were bleeding the system in the first place is to get rid of air and water!! DRAT!

Anyone who is in the business of doing alot of brake work, stores their fluid in containers which are hooked to a constant vacume, thus keeping the fluid from touching air. Them we come along with our pressure bleeders that actually force 30PSI of air pressure onto and into the fluid!

Well, sorry if I have thrown a wrench into your plans.... Like I mentioned, pressure bleeders can be great tools, but only if the brake fluid and air pressure are kept seperate.

mikey

Just to clarify, it doesn't push air through the system into the pickup tube. The pressure of the air on top of the fluid can cause the air and fluid to mix down near the molecular level.... When this happens you get very small bubbles mixed with the fluid throughout the fluid. Then this fluid goes through the system with these small air bubbles mixed in.

Since DOT 5 has a much lower viscosity that DOT 3 or DOT 4, the problem is made even worse with DOT 5. The lower viscosity makes it easier for these air molecules to fit in between the brake fluid molecules.... Yup, water doesn't mix with DOT 5, but air mixed in the system will cause less pressure to actually reach the wheels where we require it the most!

Most of us will never notice the difference, as we never use our brakes near the limit.

But if all we need to do to ensure that our brake systems are up to snuff. is to follow the correct procedure, I personally feel it is worth the effort.

mikey

Thanks for explaining. That all makes sense to me. I wonder if we could make some type of a simple bladder to these weed sprayers. Or maybe we could use another container (like a shock reservoir with a dividing piston in it).

Just out of curiosity, what's you're background that you know so much about all this?

Put a teaspoon of the fluid into a small jar of water, and then swirl it around a bit. If it is DOT3/4/5.1, it will vanish into the water without a trace. If it is silicone DOT5, it will spread out on the surface and shimmer. You will know it when you see it.

Good idea!

 

[stumps]

DOT 5 is higher viscosity that DOT 3/4/5.1 when at room temperature and above. This is easily visible in my test jar, and is confirmed by the various manufacturer's data sheets. At very cold temperatures DOT 5 is lower viscosity than DOT 3/4/5.1... a pretty desirable thing.

This viscosity data is extracted from various manufacturer's data sheets for -40C/100C:

DOT 3 1466cST/2.1cST
DOT 4 1400cST/2.3cST
DOT 5 120cST/7.0cST

Higher cST values indicate higher viscosity.

DOT 5 is required, by Department of Transportation mandate, to be chemically compatible with all DOT registered brake fluids, and all rubber brake parts. It has been tested out the wazoo for compatibility, yet the myths remain.

I have heard the spongy pedal stuff for years, but the funny thing is when I put it in my cars, the pedal becomes, if anything, stiffer. I have not observed this spongy pedal phenomenon. I must not be a connoisseur of brake pedal stiffness.

One possible cause, of a spongy pedal, could be unintentionally stirring up the DOT 5 so much during bleeding that it entrains air bubbles.

The DOD specified pressure bleeder system puts air pressure directly on the DOT 5 fluid.

-Chuck