1999, 4.0, auto. At the end of last summer my A/C went out and troubleshot it as much as I could. I could fill up the system, and it would stay cold for a day, and then all pressure was lost. I did the flourescent leak check, glasses and all, nothing up to the firewall. The compressor works, all the switches work, it just loses the freon somewhere. Are there any prone locations that leak on both sides of the firewall?

 

The a/c evaporator is inside the cab under the dash. Leak could be there.