If you have brembo brakes you cannot use dot 5 period. Silicone based fluids react with natural-rubber seals to swell them WHICH CAN CAUSE SEVERE PISTON RETRACTION PROBLEMS. Not something I would want to happen to my brakes while riding

 

Very simple answer; they are not interchangeable. DOT4 is glycol based. DOT5 is silicone based. Brake fluid is specified by brake system requirements including temperature and other parameters. Paint compatibility and the corrosive nature of the fluid are not 2 parameters considered. I represent a company that maufactures elastomer brake components. I've seen what happens when brake fluid that is not compatible with brake system seals is installed. Can you say MUSH.

There's a lot of discussion/concern over which fluid is most corrosive. What you need to be concerned about is having the correct brake fluid installed on your bike. You do not want to find out at 65mph the seals on your 900lb motorcycle's brake system have been turned to MUSH by installing incompatible brake fluid. At that point you will pray to GOD the most serious issue you have to confront is repainting your bike frame.

NEVER INSTALL BRAKE FLUID OTHER THAN THAT SPECIFIED BY THE MANUFACTURER OF YOUR BIKE.

RANT OVER

 

Dot 5 (synthetic) is best since it does not absorb moisture and DOES not hurt paint wich is good idea on hand brake lever. Five does not affect brake pedal fill. Five cannot be used in ABS units. Not sure why Harley went to 4 for newer non ABS bikes except to save a nickel or maybe since the ABS units needed it they felt all should be 4. Five is purple when new but soon changes to clear. Use what you system says. If you switch it no matter if you were to flush it, unless you change what the cap says someone else is going to put wrong thing in. Four has a slightly higher boiling point. Most would never know difference running 3 for 4 unless you are a hot-rodder in a 3 or 4 system

 

Nice new fresh DOT5 is indeed a cool purple colour. I have never used Harley DOT4, so don't know what colour that is. As DOT5 ages it turns pale yellow! If you are talking of your 2010 bike it is likely that should have DOT4 in it.

Gentlemen, allow me to chip in my experience of using DOT4 in a DOT5 bike, something I did in complete ignorance for several years with my old Evo. I didn't suffer any failures as such, the seals didn't deteriorate or swell, but the cocktail of fluids created a jelly-like yuck that eroded both master cylinders seriously. I found this while doing routine bleeding when strange traces came out with the old fluid. Cost me a total replacement brake system. So don't mix fluids! And don't trust bleeding to remove all traces of the replaced fluid.

 

I just purchased a set of used chrome handlebar controls for my 05 Roadking Custom from another member. They are for 96-07 I think. The thing is, my current stock master cylinder uses DOT4 and the new /used chrome master cylinder cover says to use DOT5. Should I flush out the little bit of I'm assuming DOT5, and use DOT4 when I switch contols? If so, what should I use to flush the master cylinder? Thanks in advance. Any input would be greatly appreciated.

 

I suggest you dismantle them and flush everything in new DOT4, including the interior of the master cylinder and reservoir. I don't see the point of using anything else. You are looking to remove as much of the DOT5 traces as you reasonably can. Reassemble and dry thoroughly, to keep any DOT4 away from painted surfaces!

 

It is worth being very specific and clear before going any further. What components - exactly - will have come from the Dot 5 system and what came from the Dot 4 system. If I am imagining this correctly, the only Dot 5 component is the reservoir itself?

 

The reservoir is integral with the master cylinder, which is under the reservoir. So there will be trapped fluid inside the piston assembly, which is where the greater source of cross-contamination is. The parts are easily removed, cleaned and replaced.

 

Fair enough, I would also want to put new seals in that were Dot 4 compatible. I would then also make sure I restamped the reservoir lid to say "Dot 4".

Incidentally, this would include the reservoir lid seals

 

Your putting used brake components on? Spend the $40 and get a master cyl rebuild kit and caliper o rings if they were contaminated. After cleaning the parts always wash out with alcohol before assembly. and use the supplied lube or a silicone based lube like silglide.