Auto Air Conditioning & Heater Repair: adding r134a, pressure sensor, accumulator

Question
I have a 2001 Boneville 3.8 SLE.  I want to top off the refrigerant and want to be sure I have the low pressure side of the system.  The line that goes across the top of the accumulator that has easy top side access (not connected to the accumulator) has 2 ports, one larger, one smaller.  Is this to accomodate 2 sizes of fittings or am I looking at the wrong port(s) to recharge ? Thanks.

Answer
Without using a set of AC gages to show you how much pressure is already in the system can be EXTREMLY dangerous and damage the system. There is a high and low pressure sensor within the lines and if you put to MUCH or to LITTLE freon in the system the computer will shut off the AC.