Depending on which compressor you have, they can be overfilled. When the clutch engauges watch the site glass, it will fill up quick, then fall a little. They say to keep filling until the site glass, stays full, when the clutch is engauged (or with just a few bubbles). But I always leave mine just a little low, pressure is probably going to go up some, as the summer progresses. An overfull system, can have a full glass, which can appear empty (freon is clear). Emptying the system (put it in vacume) and adding the appropriate wieght of freon, is always the best solution. A few odd bubbles or foam, rarley degrades the cooling much, but if much air is mixed in the system, it will push your pressures up to unacceptable levels. Always purge the lines, with a little squirt of freon, before hooking them up. Make sure the hose ends are dry, moisture will mess up the system. Putting the freon, fill bottle, into a bucket of hot water and filling slowly into the low side, only when the clutch is engauged, works for me. With the compressor clutch disengauged, the low side pressure can go higher, than the filler tank pressure, you can push freon back into the filler bottle. Only open the valve a little, so you can shut it off quick, if the clutch disengauges. Jumping the clutch or pressure switches, for continious run, is dangerous. Anything over say 350 PSI can rupture the lines (goes off like a grenade), pressure builds from 250 to 350 in seconds, if a line becomes plugged. The rated burst, is some higher than 350, but it´s good form to plan for the worst.
Most systems have a high pressure switch, that will shut of the clutch, when the pressure gets too high and/or an internal bypass. Both often make a pop. pop, pop sound, when the compressor is over full.
I often, feel the low pressure pipe, near the compressor, when it starts to get cold, thats usually enough freon or darned close.
The freon level in the system is like a bell curve, not enough freon, too enough freon (by wieght) (peak efficeintcy)and then the efficientcy starts to fall off again, with too much freon, as the liguid to gas change (peak cooling), gets pushed into the engine compartment (where it doesn´t do much good).
Filling the system on a hot day, with the doors open and the fan on high, usually works out the best. Helps keep the compressor from cycling, to often and makes overfilling unlikely. After you get, close, you can shut the doors and watch it cycle a few times.
If you have a gauge, the low pressure rarley goes above 25 PSI (R-12) and the high pressure max is around 250 (R-12), or somewhere between 200 and 250 (it changes with system load and outside air temp.). If you hook up a high pressure gauge, be carefull, the hose is under quite a bit of pressure, the liguid gas in the high pressure hose can burn (frostbite) your fingers. Keeping the gauges above the level of the compressor, will help keep oil out of the gauge hoses. The oil level, on the compressors with the freon, mixed with oil, isn´t real critical, between say 4-6 OZ. If it hasn´t all leaked out, I wouldn´t bother with adding much. You usually have to dismount the compressor and turn it upside down to get most of the oil out (of the oil integrated) compressor, the york is different (it has a seperate crankcase oil supply).
and things have calmed down now. I'll mess with it some more this weekend. Feel free to hop on the next flight to Florida and help 
