Deenybean: With the caliper off, pump the lever until all 4 cylinders are as full as you dare get them (checking the level in the master cylinder as you're doing this, no point in pumping more air in!). If you don't have any fancy tools to limit the movement of the pistons, offcuts of wood or even a pair of old brake pads will act as suitable spacers.
Now tap the caliper gently to try to dislodge any stubborn little bubbles of air up towards the bleed nipple, loosen the nipple until you get no more air out then tighten it back up.
The next stage involves pushing the pistons back in, a couple of small G-clamps helps here, and the idea is to force the fluid back up the brake lines carrying any bubbles that have got stuck in the lines up to the reservoir. Have the top of the reservoir on loosely while you're doing this, a brisk flow is what you're aiming for and you don't want it squirting over the paintwork. If it's a bike with twin discs, turn the steering to the left to avoid getting air trapped in the assembly that splits the brake lines.
Mind the reservoir doesn't overflow as you shove the pistons back, let some more out of the bleed nipple if it's getting close to the top.
You might need to repeat the process for both sides, but it eventually gets the brakes good and firm.