I am upgrading the front brakes on my 2004 Softail classic and will be replacing all of the components.

The OEM brakes use DOT 5 fluid, so that's what is in the rear brake system, which will go untouched, mechanically.

The question is about the selection of brake fluid for the new system. Do I install DOT 5 in the new front system, or convert the whole system to DOT 4?

 

As far as I'm concerned, DOT 5 is the way to go. It has a higher boiling point (500 degrees compared to DOT 4's 440 degrees), and it's silicone based, which means it won't harm your paint or eat away the powder coat on your master cylinder. Plus, it was OE on your bike, so there's really no reason to convert.

But, this is just one man's opinion, hope it helps.

 

What type of brakes are you going with? Reason I ask is because some calipers, such as Brembo has to use DOT4

 

I am switching to a pair of Brembo Calipers mounted to a pair of Touring fork legs on the OEM FLSTCI's fork tubes, working on a pair of 320mm rotors.

I'll be using the Touring's Brembo m/c's as well.

per Howard Messner & Motorcyclemetal.

 

There you go. You cannot use DOT 5. Brembo uses natural rubber seals and they will cause problems from swelling using the DOT 5.

 

Can I use DOT 4 on the OEM rear brake system?

or am I going to be stuck with 2 different fluids?

 

I've never heard that, but I also have no experience with Brembo brand calipers.

As far as switching fluids, there are a few different opinions and theories.

Some say that you absolutely CANNOT switch from DOT 3-4 to DOT 5 or vice versa.

Some say you CAN switch, but you must replace all rubber parts within the brake system, such as brake lines, master cylinder internals, and caliper piston seals.

Some say you CAN switch, and all you have to do is flush the entire system a couple times with the new fluid type.

I see people replace DOT 4 with DOT 5 all the time, and all they do is use a vacuum bleed at the caliper and run a couple master cylinders full of DOT 5 through and call it good enough.

I personally believe it is okay to switch between fluid types, but you must replace all the lines, rebuild the master cylinder with new internals, and rebuild the calipers with new seals before you can put in your new fluid. I don't think there is any way to get all of the fluid cleared out of rubber that came in contact with it, and they certainly do not mix.

You're probably just asking for trouble, honestly. If you aren't doing anything else to the rear brakes, I would just leave it as is and run 4 in the front and 5 in the rear.

 

Here is a tech page from cyclebrakes.
http://www.cyclebrakes.com/html/brembo_tech_info.html

As far as switching? I would leave the rear alone. The last thing you want to do is run the risk of mixing the 2 fluids. You can do a lot of flushing, but to me it's not worth it. The only fool proof way would be to rebuild everything like said above. Mix a little dot 4 and dot 5 in a cup and see what happens. Then you will see why it may not be such a good idea

 

I know nothing about Brembo calipers, so I will not comment on whether you can use Dot 5 or not. I am in the camp that believes you CAN switch from Dot 5 to Dot 4 (and visa versa) by doing a thorough bleed/perge of the system and there should be no need to change other parts.

 

Thanks PistonPuller!

Now we KNOW that you can't use DOT 5 in the Brembo systems.

The question about converting the OEM Harley rear brake system from DOT 5 to DOT 3-4 remains a matter of speculation.
......not withstanding the knowledge of the consequences of mixing them.