So I got a braided brake line for my front brakes to try and help them out as much as absolutely possible(already have lyndall z pads and floating rotors.) I noticed that my master cylinder said to use dot 5 brake fluid only. OK, so I went to checker to pick some up since I only had dot 4. I think I read somewhere the dot 5 is colored purple to distinguish it from non synthetic brake fluid(please correct me if I am incorrect on this). I also think I read that if you were to swap dot 4 with 5 you would have todisassemblethe brake assembly and completely clean all the rubber parts of the dot 4 brake fluid. Otherwise either the master cylinder or brake caliper will leak because of the different ways that the two fluids cause the rubber to swell(also let me know if this is incorrect.)
Now it's time to swap out my front brake line only to find a golden (what looks to be dot 4) fluid in my master cylinder. Not having the time totake aparteverything,I just emptied the master cylinder a pumped dot 5 through the system until everything that came out was all purple. Same situation for the rear.
Do you guys think that this is going to cause me problems? Is there a reason why the previous owner or shop(I don't think he did any mantiance on his bike at all, he was a take to the dealershipkind of guy) would change over to dot 4?

 

I have bought dot 5 that wasn't purple in the past. Maybe the purple was added more recently. I don't think you have a problem there. It could of been older dot5 that was in your system.

 

The difference between DOT 4 and DOT 5 is temperature rating...I think. I know that the new Harley brand DOT 5 is Silicone based. I just did the exact same thing you did as far as changing out pads and brake fluids. I just flushed the old (gold) fluids out untilI got purple fluid and I have had no problems.Your brake fluids should be changed annually because brake fluid is hygroscopic by nature and absorbs moisture from the air like a sponge. Over time, this effect will deminish braking power and internal braking components.

 

There are a few threads about this here and lots of www info out there too. There are some trade-offs to each type of fluid. HD has had some changes over the years and most recently appears to be moving to DOT4 in all models.

The highlights are:

DOT5 is silicone based and less hygroscopic so it doesn't absorb moisture from the air like DOT4. It also has a higher boiling point so it maintains its performance during heavier braking applications (like racing). DOT5 is not caustic to paint. DOT5 is not used in ABS applications. Last point is that DOT5 is not as compressible as DOT4 and others. That leaves you with a somewhat spongy feeling brake system.

DOT4 is polyglycol based. It is more likely to pull moisture from the air over time (that's whay your new amber fluid ends up turning brown/black over time). Its boiling point is lower than DOT5 but higher than DOT3 and below. Having said that, note that the boiling point referneced is the DOT standard and a lot of companies make their fluids to exceed the standard. DOT4 is caustic to painted surfaces - this is the stuff you want to avoid spilling on anything. DOT4 is more compressible as DOT5 and other DOTx fluids. That helps provide a more responsive feedback in the pedal and lever as the brakes are being applied.

I've never read or heard of a definitive answer why HD is moving to all DOT4. I know Sportsters were using DOT5 in the 04-06 bikes for certain. Not sure if they moved back to DOT4 in later years.

As far as your project goes, I'd guess you'll be fine. There is no difference in the rubber used to make seals for either application. But the fluids do not mix together (oil and water concept). As long as you flushed the old before filling with the new, you should be ok.

 

Just a note to let y'all know that DOT 5 does not absorb water. DOT 5 is silicone-based. DOT 3 & DOT 4 are glycol-based. DOT 3 contains glycol and glycol ethers. DOT 4 is made of glycol, glycol ethers, and borate esters.

The water that gets into a brake system will readily become part of the chemistry with DOT 3 & 4. It will rarely ever reach more than 3% of the mixture, and the corrosion inhibitors in 3 & 4 will easily handle that, short term. Long term, the inhibitors are depleted and corrosion then starts. As pointed out, the lower boiling point of the water can cause serious braking consequences, too.

In a DOT 5 system, any water stays separate from the fluid. The water is heavier than the silicone fluid, so it collects at the low point of the system. The water can still cause corrosion, however. Usually the silicone and corrosion inhibitors prevent corrosion, short term. But if the fluid/water mixture isn't flushed regularly it can start corroding, especially where the water tends to collect.

I always liked DOT 5 'cause it didn't absorb water, and it doesn't eat paint. DOT 4 is a better fluid for brake "feel" than DOT 5, though. Just don't mix 'em trying for the best of both worlds!