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Hydraulic disc brakes are quite common now. Before Honda came out with the CB750 in 1969 you didn't see them much, now they are everywhere, even on clutches. They work very good and are quite reliable, but are often neglected. No one thinks about them until they don't work, then they think about them alot ! Usually you go out to ride in the spring, pull the lever to stop and the brakes don't work, lock up, drag, or feel spongy. Other times the brakes worked OK, then you put a new set of pads on and the pistons won't go back in the caliper. Other times they do, but then drag when the brakes are applied. The cause is poor or no maintenance. Brake fluid loves water and when water gets in so does gook and rust. The cure can be somewhat easy or an all day affair.

Put the bike on its center stand and remove the brake caliper from the disk. Usually two bolts hold it in position on the front wheel and the rear wheel. Alot of times it is easier, on the Rear Wheel, to take the axle out and pull out the whole mounting bracket. Position the handlebars so the master cylinder is level and take a large, wet, towel and put it over the gas tank and any painted surface. Brake fluid will ruin a painted surface. Take off brake fluid reservoir cover and make sure it is filled with fluid. After removing the brake pads, pump the brake lever once while watching the caliper piston. You will see the piston move out just a bit. Put a bowl under the caliper and keep pumping until the piston pops out. Keep an eye on the reservoir and add brake fluid as needed. If there is more than one piston, let one move out a bit then put a C-clamp on it to keep it from moving out any further and keep pumping... the other piston will now move out. Alternate the C-clamp between the pistons so they move out of the caliper together equally, until they can be removed.

When the piston pops out, look inside the caliper and you will see the round, square sectioned O-ring. Remove it from its groove and clean everything in clean brake fluid. I use brake fluid because if you use solvents you must make sure you get any residue cleaned out. If you don't, it could contaminate the new brake fluid and O-ring. Remember, it's your life at stake here.
 
 

Use new parts and/or rebuild kits.

On older bikes you may have to clean and reuse old parts. If the piston has any marks or pits in it,
 

GET A NEW ONE.

Reassemble the caliper using only brake fluid for lubrication. Use your shop manual to aid in assembly... You do have one, don't you ?

With the entire system drained of old brake fluid, bolt everything back together.
Now comes the fun part !
 Fill the master cylinder fluid reservoir with new brake fluid of the right type. ( DOT 3, DOT 4, Dot 5 ) Put a hose, of the right size, on the bleed screw and put the end in a jar. Pump the brake handle or pedal several times and then hold it on. Open the bleed screw, on the caliper, and fluid mixed with air will come out. Close the screw, pump it up again and repeat the process. Keep it up until you have a firm feel to the lever and no air in the brake fluid coming out. Lastly hose everything off with water to remove any trace of brake fluid.

Sounds so easy... Yes ? It can be, if all the parts are new. Unfortunately, by the time I see these things they are pretty far gone. If you were to replace everything... caliper(s), master cyl., hoses...you could easily lay out $500 or more. Because of this we end up using alot of old parts, which is OK if we keep it within reason.

So, you pump and you pump... nothing. Now what ?

Try bleeding the air from every fitting  on the hose, such as where the hose attaches to the master cyl. and anywhere else that there is a fitting.
Try filling the system from the bleeder screw with an air pump and bleeder jar.
Tap the fittings lightly with a hammer to jar the air bubbles loose.
Let things set overnight, then do everything over again.
Be patient, keep bleeding the system... this may take quite a while. Even after lots of effort, the brakes can feel spongy. This can be from old brake lines which are expanding just a bit. The cure is new lines.

The hydraulic clutch is handled in the same way.