Why does my Mitsubishi endeavor ac work part of the time, and other times blows hot?

The compressor doesn't kick on when blowing hot. The expansion valve has been replaced already. The dealer now says some type of O-ring was leaking causing the pressure to fall - keeping the compressor from kicking on. Does this sound possible? Would it automatically build the pressure back up? Because, there was no rhyme or reason to when it worked and when it didn't.