Question: A/C issue

[A1Avette79]

I have a 79 with original A/C. The compressor was replaced in 2007, about 25,000 miles ago. I have a valve in the heater line to prevent hot water form circulating thru the heater core. I have to add freon about every three months. When the freon is fully charged the system works OK.
Lately it seems the system is not working nearly as well when I am in stop and go traffic even with a full freon charge. When moving the air is noticably cooler. As soon as I stop you can feel the air get warmer. The engine is running nice and cool - no problems with over heating. I only use the "max" setting for the A/C.
Anybody seen this problem before?

 

[TimAT]

79 system should be the CCOT system IIRC- If it is, there are a couple possibilities. One is the evaporator is clogged- usually from debris from the outside. Leaves, grass, goo all get in from the outside and restrict the airflow. The other possiblitiy is the orifice tube is partially plugged. Not real uncommon. If the compressor is (or has) ever made any noise, there's a real chance the orifice is plugged. You can figure out if that's the case, check it by running the system, and measuring the temperature on either side of the orifice. I don't recall what the difference should be, but it should be significant. The orifice is the expansion valve- one side is hot hi pressure gas, the other is cold low pressure liquid. If you have very little difference that may be where your problem is. If you break into the system for any reason, change the dryer, replace the orifice, and flush the rest of the system.

Just reread your post- if you're adding freon that often, you have a leak. Find that. Is there an oil trace on the bottom of the hood over the compressor?