Just checking why would my wifes 2005 XL1200c sporster have DOT 5 brake fluid and my 2008 FXDF Fat Bob have DOT 4 brake fluid I thought the DOT 5 silicone brake fluid was better

 

Dot 4 fluid is ether glicole based. Very hygroscopic, must be changed every couple of years. Silicone fluid is not hygroscopic but, unlike most liquids, it can be compressed and therefore gives brakes a spongy feel.
Personally, given the very low cost and hassle of changing the fluid, I'd use dot 4 as it allows applying braking force in a more measured way and better feeling (think about the difference between regular rubber and ss braided braking lines). Note that the two can't be mixed or interchanged: rings and seals and lines for dot 5 get eaten by dot 4. You should use only the recommender fluid in a given braking system.

 

thanks for the info

RENNORRIS

 

They switched to DOT 4 when they started coming out with ABS. The DOT 5 undergoes aeration in an ABS system and is not suitable. Rather than manufacture two types of systems, they just went to DOT 4 for everything.

 

From what I've read through the years, both have advantages and disadvantages. Belows are listed my opinions on the matter for whatever you think that is worth.

DOT 4 absorbs water while DOT 5 does not. Absorbed water can increase corrosion of the internal brake parts (pistons, piston bores) and can turn to water vapor if the brakes are used hard and the fluid gets too hot. Water vapor will compress when the brakes are applied and reduce or eliminate pressure on the brake pads which reduces stopping capability.

It's easier to flush air out of a DOT 4 based system.

DOT 4 damages painted surfaces while DOT 5 does not.

I have never heard or read anything about silicone fluid being more compressible. That statement does not make sense to me as fluids cannot be compressed.

I have also read that both fluids need to be periodically flushed.

Also, from what I've read, the maker of the ABS system used on HD's chose to go back to DOT 4 for their system. They would have to explain their rationale but a guess would be that they did this because DOT 5 systems are more difficult to purge the air.

 

It's not just an HD thing in regards to DOT 5 and ABS systems.
DOT 5 should not be used in any system (bike or auto/truck) that has ABS brakes.

 

The silicone fluid will absorb (trap small particles) of air. The rapid pulsing of an ABS system is likely to cause more air to become entrapped. The fluid is not compressible however the air trapped in the fluid is.

 

DOT 5 is generally used in restored older cars that get very few miles and don't get the brakes exercised as much. Some people erroneously think DOT 5 is superior in all applications and probably changed it in the sporty for that reason.

 

So, what should I use in my 07?

 

It will say right on the top of the master cylinder. I'm pretty sure yours is DOT 4, but don't quote me on that.