I'm going to be recharging the A/C in my '89 300CE. Essentially the A/C works well when the car is first started and the interior is cool ie. car has not been parked in the sun. However, once I hit traffic in the city it starts to get hot and not cool at all.

At highway speeds the performance is notably better. I'm thinking that I'm just a bit low on refrigerant. Would it be better to evacuate and test the system for leaks? Should I consider replacing the receiver dryer and some seals and O-rings? I don't want to have to do the dreaded evaporator replacement so I'm leaning towards having the dryer replaced and the system leak tested as opposed to just topping the system up a bit with R12.

Is there a chart somewhere that I can use to see what pressures and performance (vent temperatures) I'm getting depending on ambient temperature? Thanks