Remove the cylinder and brush away all external dirt. Take out the two rubber boots and extract the internal components; if the pistons are stuck, apply air pressure to the fluid inlet hole while ensuring the bleed screw is refitted and closed. Wash all components in hydraulic fluid or methylated spirit, examining the surfaces of the pistons and cylinder bores for any scoring, scratching, or 'bright' wear areas, and if any are found, renew the wheel cylinder completely. If the pistons and cylinder are in good condition, discard the old seals and boots and obtain a repair kit. Fit the new seals using only your fingers to manipulate them into position, and dip the pistons in clean fluid before assembling.