You need a pair of circlip pliers (internal type). There will be a clip, a spacer/washer, soft seal, and then a high pressure seal. The first seal is made of a very soft rubber and is easy to get out, but the inner seal is quite recessed and considerably more difficult to remove. Drill a small hole in the seal (not too far), then put a self-tapping screw in and pull the seal out with a pair of pliers. When you remove the last seal the fluid will dribble out, so put a suitable container under it and leave the system to drain down.
Les.