Hi guys, haven't been on here for a while but still lurk from time to time. Anyways, working on a friends 4450 with AC problems. Haven't really messed with the ac systems on these tractors in a long time but I noticed on the compressor it has one nipple with a schrader valve and the system has supposedly been converted to R134a as the tag on the compressor verifies that. Pump won't turn on and the lights are on in the cab so I'm sure it's low on refrigerant. It has the glass eye on the drier also but from my understanding that type is for R12 ??? I've got the gauges and vac pump and we bought the R12 to 134a conversion fittings as I only have 134A gauges....I would like to pressurize the system with shop air and check for leaks first before I waste several cans of 134A. I know to replace the drier after the system has been opened up. Any help or insight is appreciated.