Firstly, just to make sure we have our terminology correct. The caliper is the large aluminum block which sits astride the front Disk. If you don't have a front disk, then this procedure is not for you.

Inside the caliper there are two pistons which push pads against the disk, the Disk is also sometimes called the rotor. The Standard 'Lockheed' caliper was fitted both behind the right, (on earlier bikes) and in front of the left fork legs. On the outside of the Caliper there's a round steel plate with two blind holes, this is the inspection cap which allows the caliper to be serviced. It's removed by rotating it anti clockwise. Under the inspection cover sits the outer piston, the other, opposing piston is the inner piston. Both pistons are cup shaped, and sit in the caliper with the open ends pointing towards each other.

Aside from terminology, the other thing we have to ensure is that where you will be working in the brake components needs to be really clean, use a strip of lino on top of the bench if you have to, but don't work on the caliper rolling around on a dusty concrete floor or a dirty work surface. If you pick up and incorporate any dirt into the internals of the caliper, there's a chance you'll block the internal passages the fluid uses, prevent the piston from moving properly, or cause the seals to leak.

Taking the caliper off the fork leg and disconnecting the hydraulic pipe is not normally a problem, but what you have to remember, is that its a whole lot easier to remove the inner piston from the caliper if you remove the pad and pump the brake lever to push the inner piston out. This should be done with the outer piston pushed as far out as possible. Keep it's pad in position and hold it there with a screwdriver inserted where the disk usually runs. taking the pad off the inter piston allows it enough room to move.

Once the inner piston is as far out as possible, (and it can come out past the seal, so be careful to catch the fluid), you can remove the caliper from the bike and place it on the bench for disassembly.

The other thing worth pointing out, is that if its been a while since the inspection cover has been taken out, then it will be easier to remove this while its still on the bike, (although you can't use the fluid to move the pistons about when the cover is off). Commercial beg spanners are available for the job, as you can see, I fabricated my one from a shelf bracket and two bolts. If the inspection cover wont move, then wait till the wife's out and take the kettle to the garage and poor boiling water over the whole caliper and have another go. I've found this always works, the Caliper is aluminium, and the inspection cover is steel, they both expand a different rates therefore.

Removing the outer inspection cover will reveal the back of the outer piston. This piston can be removed by inserting a screwdriver and levering it out from the slot where the disk runs. You then need to remove the outer piston seal, before attempting to remove the inner piston, otherwise the whole thing just gets bound up. The seal sits in a slot in the caliper and is made of neoprene, it can be fiddled out with a sharp screwdriver.

Once the outer piston and seal are removed, the only way you can remove the inner piston is to construct an expanding tool which clamps to the the inner surface of the piston. Looking down on the inner piston from the inspection cover side, It looks like a little bowl at this point, and you need to clamp the inside walls of the bowl firmly to extract it. Last time I did this, I improvised with a Tap holder and a nut, but have used a pair of pliers in the past, bolted together with the working surfaces in side out. The tool you use must be long enough to reach down insider the caliper body, and strong enough to be able to apply considerable pressure on the piston to extract it. I found a wriggling motion once clamped worked ok. As my dad would say 'Worrying It Out. Once the inner piston has been removed, you need to remove the inner seal, again this can be removed with a screwdriver, but you need to exercise extreme caution, as you do not want to damage the internals of the caliper body.
 

This is a picture of my improvised piston extractor, a disassembled tap holder and nut. You can also see the peg spanner I use fabricated from a shelf bracket with a couple of bolts. The top pistons and the inspection cover are lying along the front of the bench.          Click on the picture for a full size image.

So, you now have a bare caliper body, two pistons and two seals, all lined up on the bench. Firstly you need to obtain a new pair of seals, these are relatively cheap and need to be replaced each time the caliper is stripped. Secondly, its probably worth replacing the pistons as well. Stainless replacements are available, which will not corrode and seize inside the caliper body.

Before you start reassembly, you need to clean everything incredibly well. I used petrol and toiled paper, rolls of the stuff, to clean out all of the solidified brake fluid and road debris. It needs to be completely clean, sorry to say it again but its important. you need to use the hole where the cover goes and the slot where the disk runs to check the groves where the seals sit, top and bottom, all the way round. Keep at it till the toilet paper is clean. I'm sure that the problems I've had with a leaky Caliper have all been down to dirt stopping the seals from seating correctly.
 

This picture provides a little bit more detail on the caliper body, you can make out the groves, and the pistons. These are Stainless ones, purchased from RGM Motors over 10 years ago and well worth the price. You can also see, behind the caliper, Dot 5 Fluid, and new pads from EBC, curtsey of Mick Hemmings.     Click on the picture for a full size image.

While the caliper body is off the bike take out the blead nipple and make sure that all the internal passageways are clear. It always amazes me that they were able to cast and machine a part with holes that run though each side.

Once the caliper body is clean and passages clear, check for any gouges or deep scratches in the bore, or any damage to the groves where the seals live. Assuming that these surfaces are ok, you can start reassemble as follows.

1. Lubricate a seal with brake fluid and insert it into the inner piston grove.
    Make sure its fully seated before proceeding or you will nip the seal and it wont hold fluid.

2. Insert the inner piston, concave side out, towards where the pads go.
    Only use your hands to gently push the thing in, it should just slide right in.

3. Lubricate the second seal with fluid, and insert it into the outer piston grove.
    Again ensure its fully seated.

4. Insert the outer piston and press it home.

Finally, replace the inspection cover rubber gasket, it sits in a grove in the cover, and screw this back in to the caliper body.

When its fully assembled, put it back on the bike, insert new pads, and fill up the master cylinder with fluid and bleed the air out.

Any leaks from the caliper at this point mean that you've nipped a seal inserting the pistons or that the pistons are scored, and you have to repeat the whole process again.

 
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