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DOT 5 Brake Fluid.....just say no!


RHill

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Short story, I'm an idiot, just say no to DOT 5 brake fluid.

 

Long Version....

 

Bought some DOT 5 brake fluid awhile back and had it sitting in the spare parts box for the longest time.

 

Did some research before I bought it and knew needed to be installed with a flush.  Well, like I said, I'm an idiot and didn't completely flush the system.  Ran a few track days with it, first two the color changed slightly from purple to grey...OK, don't really care about that.  Brakes felt a little spongy at Mid-O, so I popped the master to bleed them.  Congealed nastyness lay in the reservoir.  Dump it out, fill it up and vacuum through 2/3 of a bottle to clean out as much as I can.  As the day progressed, the master cylinder started making a squeak noise when compressed.....as I said, I'm an idiot, so I rode the rest of the day with no real issue, but the brakes felt like ass.

 

Tore the system down tonight, and that congealed nastyness was throughout the system.  The seals are all in good shape, but I'm having fun with one piston that doesn't want to pop.  Managed get a shot of brake cleaner in the eye so I'm calling it a night.  I'll be back at it again with the rear master and caliper tomorrow....this time wearing some PPE.

 

Hopefully this saves someone the trouble

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Shitty. I have some labeled DOT 4&5. Not sure what that means.

 

Don't quote me on this, but DOT 5.1 is supposed to be compatible with DOT 4 & 3.  All are glycol based, but 5.1 has a higher boiling point.  Maybe it meant to be 5.1? because 4 and 5 are not compatible at all.  If you are unsure, don't use it in anything you don't feel like fully breaking down to clean.

 

 

So aren't all bikes labeled with what kind you should use?

 

 

Yeah, more than likely it is molded right on top of the reservoir.....but as I already admitted, I'm an idiot.  One advantage to DOT 5 is that it repels water (hydrophobic), and I thought it has some magic pixie dust that might help improve my brake performance.....well if it ain't broke, don't break it trying to make it better.  

 

Got a fresh bottle of DOT 3/4 and what will be 2 afternoons into (hopefully) getting my brakes right back to where it all started; well at least the calipers are getting cleaned....one "improvement" out of this whole mess.

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So aren't all bikes labeled with what kind you should use?

Yep. Dot 5 was developed for military and race applications for heat resistance. Even at the best circumstances it has more compressability compared to dot 4.

In layman's terms: more dot does not mean more brake. Use the fluid your system was engineered for.

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Lithium battery, 520 chain, dot 5, water wetter, HID healights, integrated tail lights, PC5, K&N, ...because fucking race bike.

I'm back here with my OEM set up like :popcorn: Just waiting for the "what's wrong with my bike?" show.

Edited by 2talltim
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Lithium battery, 520 chain, dot 5, water wetter, HID healights, integrated tail lights, PC5, K&N, ...because fucking race bike.

I'm back here with my OEM set up like [emoji14]opcorn: Just waiting for the "what's wrong with my bike?" show.

Hey, I've got most of those mods....

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There's a lot of misinformation and false beliefs about brake fluids.  The old glycol-based DOT3 and DOT4 (and the newer DOT5.1 which is diethylene glycol ester-based) are "technically" miscible, but it's not recommended by the manufacturer.  The main difference as far as performance in the three fluids is the dry vs wet boiling points.  DOT5 is a silicone-based fluid and most definitely NOT MISCIBLE with glycol-based fluids.  The only time a switch to DOT5 is recommended is after a complete overhaul of the brake components, including replacement of the rubber seals and hoses and thorough cleaning of the hard parts.

 

D.O.T. Minimum Boiling Points (degrees Fahrenheit):

     DOT3  DOT4  DOT5

Dry  401   446     500

Wet 284   311     356

 

The main advantage to the higher DOT-rated fluids is that they boil at higher temperatures.  The one BIG DISADVANTAGE is that, generally, the higher the boiling point, the more hydroscopic the fluid, meaning that they absorb water more readily and their "wet" performance degrades more quickly than the lower DOT fluids, so you must flush and replace on more frequent schedule.  Fine for a race bike that gets an overall check-up several times a season, but not very good for a typical street vehicle that sees rain, humidity, and a less frequent maintenance schedule.

 

This leads some folks to believe that the silicone-based DOT5 fluid is the answer to water intrusion in the brake system.  It's true that silicone is far less hydroscopic--on the order of only a few hundred PPM vs 5-6% for glycol base--but therein lies the rub.  Your brake system is not perfectly sealed, allowing some moisture in through the master cylinder vent and even through the rubber brake line material.  When water contaminates a silicone-based system, it can't partition into the fluid, and generally settles to the lowest point in the system, i.e., the calipers.  There, it not only can cause corrosion of the metal, but the water has a boiling of....well, water, which is only 212*F.  In addition, silicone brake fluid is slightly more compressible than a glycol-based fluid.  This is why silicone brake fluid is not recommended for racing applications.

Edited by Bubba
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My mountainbike brakes on mineral oil. If I crash into a ravine and am too injured to climb out I can drink it. Plus IT doesn't eat paint and doesn't absorb water.

mineral-oil-boiling-points.png

Lol.... Not sure you would want to drink mineral oil in that situation. It has zero nutritional value. It gives you diarrhea and it prevents your intestines from absorbing certain vitamins....but hey, if that's how you get down, go on with your bad self.

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My mountainbike brakes on mineral oil. If I crash into a ravine and am too injured to climb out I can drink it. Plus IT doesn't eat paint and doesn't absorb water.

mineral-oil-boiling-points.png

If you're going to die at least you'll have lube.

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