Start from scratch. 13 year old car losing 1/8 pound per year and you are already at 1/2 capacity. PLUS you have lost the same amount of system oil (less lubriacation for the pump).

1) Buy a digital pencil thermometer from Harbor Freight for $6. You should get 40 to 44 degrees at highway speed at the center vent after the system has been running.

2) Get the car in to your mechanic, put a set of gauges on it and start there. Its probably low because of a leak.

3) Find the leak and repair.

4) Refill with more oil AND more refridgerant.

Doesn't sound like you have mcuh experience with a/c, perhaps this is something you ought to have done. Home DIY can do more damage to a/c systems than repairs if they haven't done lots of extensive research. PLEASE find a good a/c person. If the system needs a charge AND the tech doesn't add any oil to the system, I'd be careful.

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84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)