Air conditioning Q's inside

OK, so I got the little refrigerant refill kit with a gauge. I located both ports (high and low.) One is up at the passenger side firewall and is a large fitting. The other is near the radiator and is a small fitting. The fitting on the kit does not fit over the large port. The small port is at 150 PSI. So what gives? Is my low pressure side at 150 psi, meaning something in my system is all sorts of fucked up? Am I retarded and just not able to put the kit on the true low pressure fitting?