Before you do anything with the refridgerant... Was the electric fan at the condenser running when you got your pressure readings? If so your pressures are probably OK. Also...if the system is low on freon the ac compressor clutch should be engaging and disengaging frequently.
I'd pinch off your heater hose to see if there maybe a problem with your heater (water control or actuator doors). The heater only needs to put out a little heat to completely mask any cold the ac is putting out.
That's my opinion anyway.

____________________________________-

83 300sd
3 ford
Ase cert. in ac