Automotive Air Conditioning Systems That Will Not Accept Freon | eHow.com
Here is a brief summary of causes of AC system not allowing 134a into the system.
There is a lot of specific advice on the web. That is how I learned to locate and replace my AC compressor last summer.
Note: If you have the Over-The-Road bus air feature providing AC in the bedroom from the engine compressor, there are two driers in the system that need to be replaced when the system is opened to the atmosphere.
One next to the engine and one in the cabinetry above the bed.