| I've just had the very problem.
ie: master cylinder seems to be knackered.
What I did was filled a bottle of dot 4.
Removed the brake line from the caliper and put it into the bottle and held it above (2in) the master cyclinder.
I then pumped the master cyclinder a few time checking the oil level did not go below the min mark.
You should see air bubbles coming from the hose.
I then quickly attached the pipe to the caliper and used a brake bleeding kit from halfords.
Worked for me.
How much have people paid for brake calipers. I've been charge £80 for a ST4 rear with pads.....! | 
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