Front Brake Caliper

First of all, try to get the pad and piston out by disconnecting the caliper from the bike, removing the two bolts holding it together. Use the brake lever to first press out the brake pad and next the brake piston. When it comes out, you are likely to spill brakefluid on the floor, so place something beneath to take care of that.

If necessary, refill some brakefluid in the master cylinder during this operation. This usually works, but if not, the following procedure should do it.

1. Remove the caliper from the bike by disconnecting the brake fluid pipe and the two bolts.
2. Remove the brake pads and inspect them both for wear and dirt.

Caliper house with the piston top in the center to the left, right item is brake pad. Be carefull with the white plastic ring, might easily crack. New ones comes with new pad. 

Plenty of old residue in the housing.

Do not press the piston top down in the caliper house!!!
We use airpressure to get it out and need the space behind.

3. Use compressed air into the hole where the brake fluid pipe is connected. 3-5 bar is enough

The piston will usually shoot out easely, but be careful; to avoid damage on the piston and the surroundings, take a cloth in your free hand and hold it over the caliper. Quite fun, actually :-)

Warning:
As you press the trigger, brake fluid residue will spray out where your airpistol connects. 
Watch your eyes and don't wear a white shirt!

If the piston wont move, spary some WD-40 around the piston top, penetrating down between the piston and the caliper cylinder. Give a few very, very light knocks on the piston top with a hammer just to loosen it up. Don't knock it all the way down!

And this is what it might look like...

 

4. Clean it all up and you have inspection time. Prey out the black rubber piston sealing carefully, we don't want to have any scratches inside the caliper housing. It is impossible to inspect the upper part of the piston seal groove, so I use a dentist mirror.

 

Find a proper tool like a screwdriver or similar and carefully remove the resedue.

 

To clean the upper part of the groove, I made a little tool out of my daughters hair grip. Bend the flexible piece of metal so it  can access the groove properly. Finally, spray WD-40 into the groove and use some fine steelwhool to clean out the rest. 

I use my dentist mirror frequently, checking out the groove.
This looks better!

5. Install a new piston seal
   Pictures comming, waiting for the seal.....

 

6. Piston sometimes have rust spots, like chrome pealing off. These spots may damage the piston seal as the piston moves when you brake. Replace the piston if it is rusty. On this picture you will see some small rust spots about 5 mm from the top on the right side. I decided to let this piston pass the inspection.

 

I will continue later with installing the piston seal and the piston. You will need brake grease.