Brake problem

Hi folks,

First post here, I was asked to change a master cylinder today on neighbours octavia (2000) 1.9

The rear brake pistons burst a couple of days ago, he got them replaced, the brakes needed bleeding so another neighbour had a go and accidentally broke the bleed nipple off the Master Cylinder, he was left with very poor, to no brakes..

I removed old Master Cylinder, popped the new one on, bled it, and asked him to pump the brakes.. He pumped a couple of times, then bang (very loud pop), no brake pressure at all..

I removed Master Cylinder again and found the brake servo pin thingy to be sitting under the MC piston, so put the servo pin thingy back into the piston on the Master Cylinder and replaced again, this time he said the peddle was perfect, so I proceeded to bleed the rear brakes and bang again, this time the pressure dropped to nothing and there is a very audible metal on metal noise when the peddle is pushed...

I think it's the pin in the brake servo acting up, but what's it doing? Anyone experience this before?

Really appreciate some advise please, if you understand what I described..