Disco 2 brake pedal going hard

Just had a drive and my high milage disco, was going well but i braked going down a hill and i felt the pedal and it was hard. The next time was ok then braking at a junction it was very hard as if i had no servo!!

Question is has anyone had to change a vacum pump? if so how do i test and what figures should i see. Or is the servo going to be a problem if so why is it intermittent and how can i test???

Sounds like a loss of vacuum, run the engine and take the pipe off the servo and see if you have vacuum there, plus check the hose for breakage/splits and see if the non-return valve is working correctly.

Assume you have a diesel?

I've pulled the pipe off the servo and I am getting some suck!! but i am unsure if it is enough vacuum. How much should i get ?