Pistons & Head

Here is the head (can be lifted onto its end with one hand :-) ). I'm no expert unfortunately, the valves look pretty good, but many of the water ways are blocked with a thin layer of "something", looks like metal or could be kalc? 

Blocked - Why is this? weak water pump?

Number one piston below

Number two

Number three - something bottom left corner?

Is it worn?

Number 4

Anything that could contribute to low pressure in 2 & 3?