Komatsu PC45R-8 Weak boom

[Yeats]

I am thinking of buying a neighbours Komatsu PC45R-8 Excavator. He tells me the main boom cylinder is weak (but not the others). He has rebuilt the cylinder but has been told that is a most likely a valve problem and that the valve will cost over $1000 to fix. I know nothing about excavators but have a reasonable mechanical knowledge.
So, any ideas? What does it sound like?

Also, is there a way to stop oil from pouring out if I start dismantling valves/hoses. ie, when are things under pressure and what is the best way to relieve any pressure in the system? (I hope that makes sense!) Also, if I work on the hydraulics, do I have to bleed air from the system afterwards? How?

Aah, serious newbie alert siren going off here!

 

[DoyleX]

I have the exact same machine doing the same thing. Ill let you know what I find this winter. Or visa versa. Talked to a old kmartsu mechanic. Said if all the other functions are strong might be the pilot pressure but more than likely a poppet in the valve spool for that function.