First you need to figure out if the brake itself is sticking or its just the lever. Then you need to flush the system. Hydrolic systems on a motorcycle the fluid should never be neglected to the point of past a light tan-or weak tea color. Once it goes brown, or worse yet black you have all sorts of contaminates in the fluid-dirt and water mostly. These are the things that damage the systems-the pistons in the caliper and in the reservoir.
Once you have diagnosed if it is actually the lever or the caliper that is sticking-take that object apart and clean it with brake cleaner fluid, dry the parts, then lubricate with fresh fluid and reassemble. If the parts look pitted or rusty, better to replace them, than try to clean them up. If you do not know what you are doing, you can go too far with cleaning and create more of a problem.
When you flush the system, you will need some sort of container-old coffee can works well and a tube that fits snuggly over the nipple of the brake bleeder. Start with a bottle of Castrol GTLMA brake fluid-it is the only one that I am aware of that will blend with all manufactors formulas of brake fluid. this is important, because if you guess or buy the cheap stuff, thinking it is all the same, you can end up deteriorating the system from the inside outward. With GT LMA fluid you save yourself that headache of replacing all the rubber brake line, as well as internal parts in the caliper and reservoir.
Now put about 1" of fresh fluid in the bottom of the clean coffee can and remove the resevoirs cover. Try to do this inside a garage or on a day when there is no chance of rain and no wind, so contaminates do not blow into the resevoir. Now loosen the bleeder valve on the caliper and pump the brake lever. As the fluid gets low in the reservoir, refill it and keep pumping the lever till all the old fluid is out and in the can. Afterwards, tighten the bleeder valve and remove the hose. Pump the lever a few more times to make sure you have sufficient pressure in the system and no air bubbles coming up. When that is done, put the reservoirs cap back on and tighten down. The flushing is done and the sytem should be like new
If you find you still have a problem with the lever sticking you did not catch all the problems. and you will have to start diagnosis again and then refill the system.
One note of caution-brake fluid is very caustic to painted surfaces, so make sure you have a bucket of water and rags near by to wipe it off IMMEDIATELY if you notice any getting on the painted surfaces.
Hope this helps
