True, the freon bottle only works when the AC pressure is still

to make AC fully charged, you'll later have to turn on the compressor, which will generate a sucking power at the low pressure side. Follow the instruction and use your Manifold Gauge Set to read the pressures (hi and low). It can't be 0 after a charge of a can of freon through the high port, otherwise, it means the pipe isn't a 'closed' system any more. Whatever got charged in leaks out right away.