Can anyone tell me where the "low side service port" for the A/C is on a 1987 E350 with the 6.9 diesel? Is it on the driver's side in the line going to the receiver/drier near the radiator?

 

It is the large metal tube going from the compressor to the firewall. There should be a blue or black cap on it.

The smaller tube that goes from the compressor to the condensor at the front is the high pressure line.