Thanks everyone for taking the time to respond to my query, much appreciated!
  The kart is stored and transported in a horizontal position.
  I'm using DOT 5 (silicon) as recommended and supplied by Spellfame, although there does seem to be some discolouration of the brake fluid in the master cylinder.   I do have a proper brake bleeder, but not used it as yet - not had the confidence to use it.  Will look out the guide I printed off the web and give it a try.
  Brake pads are about 50% worn...I've never replaced these as yet – will replace these before the next outing.
  The master cylinders and pistons look in perfect condition.
  Thanks again everyone!
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