I don't know if the pressure pump would fail...if you look at the pump you can see the motor at the bottom, the plug is clearly marked + % - so very easy to run up the pump, just don't leave it running too long as you will not have any pressure switch to turn it off.
I doubt it is that.
Then if you look at my recent pics you will see the master cylinder with the gold plug out with the metal "thingy" it is my guess that this may be the problem.
I am no mechanic and I cannot be liable for any help I give but if you want to post me the pump and master cylinder I will strip it all and check what I can for you. I will bench test all mine soon so I am happy to do yours and I have learned a lot recently. I also have a complete spare which I can compare to that I will not be parting with. They do corrode inside with a furry white build up but it is easy to clear.