I accidentally topped of my front reservoir (05 ultra) with dot 5 fluid (it requires dot 4) it was only a very small amount maybe a cap full. Should I now flush the system with new fluid or will it be OK?

 

Hi from the UK and welcome to HDF.

They don't mix, because they are very different fluids. I strongly recommend flushing through with the correct stuff.

I have bitter experience of mixing fluids in my 1990 bike (owned from new). After many years of ignorance or innocence and using what I could find I discovered my master cylinders were both seriously corroded inside. Had to change the entire brake system!

You shouldn't have those sort of problems, but for peace of mind....

 

Dot 4 and Dot 5 brake fluids are chemically incompatible. Put together they congeal and swell rubber brake lines and seals in your brake system. Basically, no matter what the brake master cylinder says you can run either one but you certainly can't mix them.

I've seen some folks, who made that mistake, run enough of either fluid through the brake system in order to completely replace the mix, but if it were me I first run denatured alcohol through it for a complete flush and then refill and bleed with whatever brake fluid you finally decide on.

In any case get on it. The longer the mix is in there the worse it gets.