The cycling on/off is generally indicative of a low charge, but why it won't accept any more refrigerant is odd. Are you using a charge kit with a guage?

You might try disconnecting the plug at the pressure sensor and jumping the contacts in the plug. Then see if the compressor will run steady and give you cold air at vents. You don't want to run the compressor this way TOO long just incase it is actually low on charge, but you should know within about 10-15 seconds if you are going to get cold air especially if you run the RPMs up to about 1500. If it seems to give you nice cold air at that point, you may have a sensor switch that is out of adjustment. I've heard that some of them are easily adjustable but I have never done that myself.

Another possibility is that it's OVERcharged, as R134A supposedly has a narrower pressure range that it performs well compared to R12. But I don't know if the pressure switch opens in an overpressure situation. I'm guessing there is just some reason the Refrigerant won't flow from the can into the system, like some sort of obstruction or some little gizmo missing from the valve or your charging hose. Did you try running the RPMs up a little while charging in order to drop the pressure on the low side of the system?