1997 Chevy Lumina Fan operation

When I use my A/C sometimes the air blows through through the vents and at times it blows primarily from underneath the dash(ie on my feet). The air is not very cold and I imagine it needs a freon charge, but this problem is different. Also I know where the low side service valve is but what is the small valve toward the firewall that is clearly marked "evaporator service port" and has a note of a "1psi maximum pressure"?