davey,
Is your left shoe wet and smelly? If so, it's the master. Or, when you lay under the slave cylinder and peel back the boot, does your eye suddenly sting like heck? Then, it's the slave. Could even be both. Then you have sting and stink...
Seriously, when these rather simple hydraulic systems begin to fail, a leak is all but certain. Don't forget to check the hose and fittings as well for loss of fluid. Any help?
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon