It’s not a DIY job for most, and just by the fact that you ask these questions, you should take your car to a AC specialist.
You need very special machines and tools, pressure gauges, vacuum pumps to check for leaks etc etc.
It’s the only job for which I take my car to a repair shop.
Does it blow any cold air?
Is it still running on R12?
If so you should consider reverting to R134a…
There are R12 compatible DIY charging refrigerants but you must be sure you have no leaks.
Search the archives and the interweb, there is a plethora of info.
I second Aristides, Stuart - it’s not a run of the mill DIY…
The refrigerant level is checked at the sight glass at the end of the receiver/drier - the crosswise tube above the radiator.
Engaging the AC there might initially be a few bubbles showing - then, with a ‘full’ system there should be no bubbles. Constant stream of bubbles means ‘low’ - so no bubbles means either ‘full’ or ‘empty’.
A shop will not charge an arm and a leg for a plain refill, but they may suggest elementary checks for leaks. Needless to say; with a substantial leak a refill is just a temporary relief…
If the sight glass show oil film the system should really be refurbished - a job that indeed requires some expertise and equipment…
A crude test; notice drop in rpms as the AC is engaged; with the engine in gear, the drop is some 200 rpms with a ‘full’ system. With a depleted system the drop will reflect the fill level - some 50 rpms roughly means ‘empty’…
Though I actually have seen DIY AC refill kits (even for R12) at Wallmart when I was in the USA, I would strongly recommend against tinkering with the AC system yourself.
Especially R12 systems are to old to mess with nowadays, if you are not a pro.
Conversion to R134A would be a viable option, but requires a new filter/dryer, valve and line connectors at the least. Or use a different, allegedly R12 compatible universal AC gas.
The AC is not filled with fluid, it has a gas / oil filling.
The gas is the refrigerant, the oil for the lubrication of the system.
Generally, the only way to see if there is to little gas in the system, is to drain it with an AC machine and see how much weight of gas came out.
Knowing that, you could then refill with the gas you extracted (and a little more, in case there was to little in the system).
From my own experience I can tell you (I had one of my own cars professionally converted from R12 to R134), if you don’t change out the Whole System you will be disappointed in the result of R134. I was.
To do it “right,” one must change out not only the compressor, dryer, expansion valve and hoses, but condenser AND evaporator. In short, replace the whole system with one Designed for R134 and then it might work as intended.
“Allegedly Compatible” R-12 replacements (Propane/Butane mix, HC-12a) get a bad rap a lot of the time, mostly, I’ve noticed, from the PR department of R-134 patent holders.
I can only tell my own personal experience with these Elderly AC systems, HC-12a requires less to charge, cools better, creates lower head pressures and leaks less than even good old R-12 Freon (which I don’t think is even available anymore unless one finds a stash hidden in a cellar somewhere). So there’s less stress on the system and components last longer.
I’ve used it for well over 20 years in OEM R-12 systems (didn’t change ANY hardware) with very good results and great satisfaction. It comes in single charge cans with directions for installation on each can.
It’s like everything else in life: It’s easy if you just follow the directions. You only get into trouble when you have a better idea.