When there pressure is so low that that the system will not engage, then it's not just "low" on refrigerant, it's empty.
It sounds like you could use a good tutorial on how the A/C works. There are books available at autopart stores that can provide this. You should also be able to find something on-line.
One fundemental point to remember is that the low side pressure and temperature should always be at the boiling conditions of the refrigerant. In order for the pressure to drop below the trip conditions of the cutout switch, it either has to be sub-freezing or it has to be empty of all liquid refigerant.
A/C systems require the movement of lots of liquid refrigerant. There are two reasons for this (1) the boiling of a liquid absorbs heat (in the evaporator) without changing the temperature of the liquid, which ensures efficient heat absorption (2) The amount of heat move depends on amount of refrigerant MASS (not volume) that is moving through the A/C system.
The '96 Ford Windstar A/C system with the auxilariay unit requires 54 oz of R-134A. This is about 4 1/2, 12 oz cans. Even the system without the aux unit requires something like 40 ounzes, which is over three cans.
When you only have one can of liquid refrigerant in the system, the flow rate of liquid refrigerant into the evaporator is low, and all of that is evaporated before much air is cooled. Once all the refrigerant is evaporated, then the air heats up the gas in the evaporator without cooling the air very much.
There is only one reason that your system has low refrigerant... a leak. You need to find the leak to avoid repeatedly recharging the system. If it's a big leak, you might be able to hear the gas escaping. If it's a small leak, then the easiest way to find it is to put some flourescent dye in the system when you recharge it. As the system is used the dye will leak out with the refrigerant, and a UV light will help you locate the source of the leak.
If the system leak is large, then air + water vapor is in the system. When this is the case, the accumlator/dryer has to be replaced, and a hard vacuum has to be pulled on the system before it can be charged.
Also, they recommend that you add 2 oz of PAG oil on recharge whenever the system charge is lost
A couple of links:
aircondition.com
ackits.com
autoacsystems.com
hvacr-tools.com