Just a thought Sam
It may help if you remove the pipe frim the master cylinder & fit a blanking plug in the hole .... An old fitting & piece of pipe should do the trick ( Just fold the end of the pipe over a couple of times, then crush in a vice )
When you press the pedal, if the master cylinder is ok you should be able to feel the pressure through the pedal .
It has been my personal experience that some master cylinders don't like to be left empty for any amount of time.
Hope you get it sorted