I've been riding bikes for almost 40 years. Done a lot of wrenching; rebuilt an old Honda once, assembled a Bultaco from a foot locker full of engine parts, a shop manual, and an empty frame, spent many years in the sport of dirt racing and did all my mechanical work out of an airconditioned trailer in the race pits. So flushing brakes, replacing pads, rotors, etc is old stuff. During the last ten years of the dirt racing days I had a brake sponsorship so I would change brake fluid and brake pads before every race. We raced every other Sunday...a 50 mile off road course on Honda XR600's...two bikes - one set up for sand and the other for hard dirt & rocks. So the mechanical goal was to keep the bikes bullet proof. My routine was to add some food coloring to the new brake fluid ( DOT 4) so I could easily see when the fresh fluid made it to the bleeder valve, alternating red and green with each brake flush.
These days I have two bikes in my garage...a BMW touring bike that uses Dot 4 and a Road King that uses Dot 5. So earlier this week I had set aside some after work time to flush the brakes on the bikes. I started with the BMW, drained the front master cylinder with a large syringe, filled with fresh Dot 4, added a few drops of red food coloring, and bled the brakes till fresh red fluid flowed from the valves. Then I moved on to the 03 Road King, and started on the front brakes.
I drained the master cylinder, topped off with fresh dot 5, which is factory-dyed purple for quick identity, and started the bleeding process. After an entire master cylinder of dot 5 had bled through the right side I was still seeing the old, yellowish, fluid. So I added several drops of dark blue food coloring to the master cylinder and gave it a few pumps. As soon as I did that I realized my mistake. I completely forgot that the silicone based dot 5 fluid does not mix with water. But old habits die hard and that one ended up costing me an additional hour in the garage. Even though I drained the master cylinder of the mixture and cleaned it out before adding fresh dot 5, I had already pumped some of the blue food coloring, which had settled in little globules at the bottom of the master cylinder, down the brake line. It took another hour and a pint of fresh fluid to flush all of the water based food coloring, which separated into hundreds of tiny globules during the bleeding process, out of the brakes. After I was satisfied that the job was done I called it a night. But before I closed the garage I did an experiment. I combined equal amounts of dot 4 and dot 5 in a small container and set it on the workbench. Then I got cleaned up and had a cold Skyy martini.
Lots of internet threads claim that these two fluids will chemically react to form a sludge or gel if even a drop of dot 4 mixes with dot 5 so I was interested to see what would happen. 12 hours later, this morning, I found the mixture to be the same as it was last night. Just a separation similar to water & oil. The container, however, had a nice coating of the silicone based dot 5 when I cleaned it out as it had completely displaced the glycol based dot 4. Of course that's no indication of what might happen if heat had been applied, such as in extended use or braking conditions. But it was interesting to see how the fluids combined. I might try that experiment again with the mixture in a Folgers coffee can and apply some torch heat. Did that once with Mobil 1 and Amsoil and got some interesting results on boiling points.
I plan on riding the Road King a few times, log about a hundred miles, then do a complete flush of the front brakes again just to be sure I've gotten all of the water from the food coloring out of the system. It was only several drops so we're not talking about total contamination.
So, in the end, a good lesson about old habits was learned. And another about Skyy vodda. Maybe I should have had the martini BEFORE I started the brake bleeding job. Then I would have probably just blown it all off and enjoyed a night on the patio.

 

Interesting read! We have both been around bikes a similar time and I have first hand experience of the jelly-like sludge - it is my personal brake fluid horror story! I only discovered quite recently that there is DOT5 and DOT5.1 and they also are not compatible! I used both during the early years of owning my 1990 FLHS (bought new).

To cut a long story short I dismantled the handlebar cylinder to overhaul it, to find grey jelly and serious corrosion of the bore of the master cylinder. I stripped the rear and found the same. They were so corroded that I didn't dare rebuild them and replaced both with new. I had already fitted new front discs and callipers, rear disc and braided hoses all round, so replaced the rear calliper as well.

This was all in preparation to ship the bike 'back home' from the UK to visit Harley's Centenary in '03, so the bike had to be in tip-top condition! The trip was great and we travelled 3,500 miles before going back home.

I now buy only H-D DOT5, to be certain of getting the right stuff!