250-500cc Front brake repair.

Hi all first post here and I'm after some help, I have a 1998 WR250 and the front brake is not what it should be, the lever will under pressure come back to the handlebar with no brake fluid leaking out anywhere, if I release the brake and grab hold again the pressure has returned, I am assuming that the rubbers in the master cylinder need replacing but for the life of me I can't find a repair kit for this model, the parts listing only shows the master cylinder and not the internals, can anyone shed any light on this? I have an MOT due in a couple of weeks and the bike is going to fail with the brake like this. Other than the brake the bike rides real good, still crisp on the throttle.

Many thanks

Crappy