I usually empty the reservoir first with a bulb type vacuum pump like a turkey baster. Put the removed fluid in a clean jar of some type to be repurposed as explained below.
Wipe out any crud or settings from the reservoir until it is clean.
Then fill the reservoir with fresh fluid from a newly opened container. Never use fluid from a previously opened container as it is hygroscopic (absorbs moisture from the air). I recommend Dot 4 of any brand, and synthetic is just a marketing ploy. It is all synthetic, and either it is Dot 4 or it is not.
Connect a rubber hose to the bleed port, and place the other end in the old fluid in the jar. Then open the bleed port and start pumping the brakes. If you keep the end of the hose in the old fluid and the jar below the bleed port, there is no need to close the bleed port every time that you pump the brake. It will not suck in any air, and will pump out more fluid than it sucks back. Just be sure to keep the end of the hose in the old fluid at all times.
Continue pumping the brakes until clean clear fluid comes out, and then some more to be sure. You will have to keep adding new fluid to the reservoir to keep it full and prevent pumping air into the system. Be sure all the old fluid is out.
Don't worry about wasting the fluid by pumping new fluid through the line after it is mostly clean. You will have way more than enough fluid from even the smallest size container of brake fluid. Unless you have another vehicle that needs new fluid that day, then the left over fluid is trash. You should not save it as it will be no good after it sits and absorbs moisture.
On the last pump of the brakes, hold it, and tighten the bleed port before you release it so that no old fluid is sucked back into the line. Your brake system should now be clean, and free of air with no additional bleeding required. Just top off the reservoir to the full mark, close it up , and go for a nice ride.
1999 Honda Rebel CMX250C
2003 Harley Sportster 883 Hugger
1998 Harley FXDWG Dyna Wide Glide
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