Totally unlike any other subsystem of your car, the a/c system is not something on which you perform preventive maintenance. If it still works, and is fully charged, DON'T MESS WITH IT.

About the only thing that could be called PM would be keeping the condensor and evaporator fins clear and not bent.

What happens when you connect gauges to an a/c system is that you stand the chance of introducing moisture. Moisture is the enemy of your a/c system internals. Any moisture combines with the refrigerant to produce an acid that will eat everything up from the inside out.

Don't mess with it,