A/C refrigerant recharge

[jchu]

I am having trouble recharging A/C on my 944S2. I assume that the low side port is along the driver side fender between the sighting glass and along the line going into the dashboard. The port is pointing up with a black cap. I used a can of R12 and an old recharging kit. I run the car with A/C on and connected the charging kit. Nothing Happened!! Freeon is not going in!! What could be the problem? Am I using the wrong port? I do not see anything to push the valve down after connected. Is the freeon pressure in the can supposed to be high enough to get into the A/C system?
Thanks.