You say you have checked the rear wheel cylinders, did you check the front as well? In addition, have you checked all the associated brake hoses to ensure they are in good condition? Have you also had a brake fluid change at any time?

If all of the above is good then we would suspect that the brake master cylinder may be the problem and as such would suggest that you get it checked out at a local Good Garage: good-garage-guide.honestjohn.co.uk/