Chevrolet Astro 1991 air condition diagram!?

Hi, my Astro air condition has to be recharged twice a year , i recharged the system before 5 days but i found that the gas disapears much quickly than before. How can i get a diagram for the air condition system which it has a rear unit and whether the system has an expansion valve cause i have a suspesious about it cause i found that there was a pressure of 25psi of gas that was still from the last year before i recharged it this year .But when an excess pressure such as 40psi in the system ,the rate of dispearing will increase(accelerated disapear) about (( - 6psi/day)/day). Can i find a web site that has a diagram of this system?thanks