The spring you should be able to see from inside the car. Just a long shot that it may be broken, it is the spring that makes the pedal return. Free to have a look.
The master cylinder is seperate to the brake master cylinder, two seperate items but both share the brake fluid reservoir.
In fairness it could be either the master or slave cylinder, not sure how you would check to see which one it is unless you can see leaking fluid. Leaking fluid may be internal past the seals. Have you disconnected the slave cylinder from the gearbox to see if there are any leaks into the gearbox.
Paul