1996 altima A/C not working

[inyourneck2004]

towards the end of this summer my A/C on my altima would blow cold air for a few minutes then it wouldnt blow any cold air.so i put some refrigerent in it and it was still doing the same thing.so i took it to a mechanic and he said something was more or less stopping my system up and i would have to have my whole system took apart and everything cleaned in it inorder to have it fixed.now it dont blow any cold air at all.this may not make any sense to anyone but i tried.do i realy have to have the whole system took apart or could someone just put a machine on it to pull everything out of it.

now my second question is where is my pcv valve and how do i get to it to change it????