What year is your servo?
I have one from my 69, it was working when I took it off. I had to change it because I have put discs on.
You can have that for the cost of P + P.
Or you could try a 70> non servo master cylinder, I can't see why it wouldn't work. Only £30 form GSF. And fit straight on.
If you are woried about the residule presure you can swap the valves over from the old servo on to the new servo.
Sorry if that is a bit vague, or if your bus is earlier.
Feel free to ask me any questions...