i work in the aircon/refrigeration trade just some advice to help you guys out when fixing your car a/c
most common mistake with car air conditioning is over charging. alot of mechanics dont fully understand the basic principles of an air con system. their approach to a system that isnt working properly is there must be a leak so they top up the system. an overchaged system is just as bad if not worse then an undercharged system. the best/simplest way to charge a system is to weigh in a charge. first find out how much gas the system is designed for, second reclaim the refrigerant out of the system if there is any left in there, this requires a recaliam unit and reclaim bottle. most of you will just release it to the atmosphere. then if you had the system open to change a valve or whatever moisture would hve gotton in the system you need a vaccume pump and vac out the system for about 20 mins the longer the better you cannot over vaccume a system. then get your refrigerant cyclinder put it on some acurate digital scales and weigh in the charge. start off charging with the car off on small systems the vaccume should be enough to pull the charge in without having to run te compressor. if it doesnt fully charge run the compressor and charge a vapour into the low side of the system(some refrigerant bottles have one valve some have two, single valve bottles are usually vapour only and the dual ones usually have a red and a blue valve the blue is vapour). once thats done you should be set.
you cannot properly charge a system just using gauges and charging to a set pressure because the pressure of a system is not a constant. things like heat load in the car(people) temperature of the day affect the system pressure. you can approx charge the system using gauges but it will never be perfect. if your going to liquid charge make sure its in small squirts. soapy bubbles are a good way to find leaks.
leebo when you said you charged your system your compressor locked up, you may have liquid charged the system on the low side the compressors cant pump liquid and this can damage them very quickly.
the expansion valve and compressor seperate the high and low sides of the system the evaporator is on the low side and the condenser is on the high side.
the expansion valve is what regulates the the amount of refrigerant into the evapourator it has a small oriface inside if a system will get a block this is where it will be stopping or restricting the flow to the evapourator giving no or stuff all cooling.
some systems have safety devices in them to protect them from damage most common are low pressure and high pressure cut out switch's so if your compressor isnt running when it should this is most probably the cause.
moisture is the biggest killer it reacts with the oil and refrigerant and creates acid and starts lots of preblems.
hope all this didnt confuse i tried my best to explain it in writting. air con systems are not easy to work on without the right know how and the right tools.
if you have any q's id be more then happy to help you out.
one last thing releasing ozone depleting refrigerant into the atmosphere is illegal well in australia anyway and id bet the laws the same in america so if your gonna be releasing refrigerant dont do it in the front driveway for all the neighbous to see cause it usually incurrs big fines