Hydraulic fluid breaks down over time. Vibration, condensation, and heat all take their toll. Over time the fluid turns dark from the break down, and small bubbles form in the fluid. As air compresses, over time the clutch slave cylinder moves less and less as you pull the lever. The clutch may start to drag, i.e. you pull in the lever but the bike still wants to move forward. As the fluid ages, your clutch may stop working completely on a hot day: the broken down fluid, no longer under pressure due to the air bubbles, will overheat and boil. This will cause the loss of all remaining pressure, and the cutch will not disengage until the bike cools off.
Change the fluid. The owner's manual says to change it every two years, but I personally change it every year when the tag is due, because I ride a lot of miles.
Start by loosening the two allen bolts that hold the master cylinder to the handlebar (you may have to use a small screwdriver to remove the chrome plastic caps). Rotate the master cylinder and move it so that it sits level. Cover the tank with a towel so you don't get fluid on the tank, or drop a screwdriver and scratch the paint.
Remove the two screws and pull off the master cylinder cover. Remove the rubber diaphragm gasket. Clean both well, as there will probably be a bit of junk on them from the old fluid breaking down. If the gasket is deformed, ripped, or swollen, buy a new one (Suzuki part number 59667-49460, about $5)
Remove the chrome clutch slave cylinder cover from the left side of the bike. It is held on by three bolts, and each bolt has a tube and a rubber gasket. They may come off when you pull the cover, so make sure you put them back together when you put the cover back on.
I use a little bleeder I got at the Pep-Boys, but a length of rubber hose and a small cup or bottle will work just fine. Pop open the little rubber cap off the bleeder on the slave cylinder, and slip the hose over it. The other end of the hose goes in the bleeder or jar, with the end submerged in fluid.
Use an 8mm open end wrench to crack open the bleeder, so the old fluid slowly drains out. Keep draining until the master cylinder is almost empty, but don't let it get so low that air gets in the holes at the bottom.
Add some fresh fluid to the master cylinder to fill it up. Suzuki recommends DOT 4 fluid. Do NOT use DOT 5, as it will eat the rubber diaphragm gasket, causing little bits of rubber to clog up the master cylinder. If you want more protection from heat, use a synthentic DOT 4. I do not have a favorite brand, but make sure you do not mix synthetic and non-synthetic.
But what if...
Oh, you let the fluid get too low in the master cylinder, and sucked air into those little holes? Or maybe you have bled and bled the clutch, but the lever is still soft?
No problem. Take a 14mm wrench and put it on the bango bolt, where the line goes into the master cylinder. As you slowly pull in the lever, crack the bango bolt. This will allow the air trapped int he master cylinder to escape. Tighten the bolt before releasing the lever. Repeat one or two times, then tighten the bango bolt nice and tight. The clutch lever will more than likely be firm now, but if not bleed they system at the slave cylinder again until it is. Put everything back together and enjoy your ride.
*Courtesy of Dr. Bob's wrenching tips found here.