I replaced the a/c system on my 88 F-150 in the fall, and ever since I did that I've been having problems with my evaporator freezing up, and the other day I noticed the suction line was frozen pretty much all the way back to the compressor which obviously isn't good. I drew a vacuum on the system prior to charging, and the pressures looked pretty good. Could this be caused by the freon being low, but not low enough to kick off the compressor? I was charging out of 12oz cans which always feels like a bit of a crapshoot for getting the system filled just right.