You can rent most a/c tools from Autozone, Advanced Autoparts, or Pep Boys. However, I've found it less expensive to simply have my local NAPA shop hook up their a/c machine pump out the system and refill it for me! Unless the system is fully and effectively evacuated, they simply don't work or work well. Also does your car have the splitter valve or the little plastic piece that goes in the a/c line to split the gas into liquid? If you have the plastic piece it usually has to be replaced along with any necessary o-rings. If the system isn't holding freon pressure and converting it from gas to liquid the sensor switch(es) won't kick in. Sometimes when you have pressure you have to by pass the switch to see if you can get the compressor to kick in and if it will start converting gas to liquid. There are basic a/c books sold at Autozone and a CD tutorial on a/c system repair that shows how to use the gauges to diagnose the system. See You Tube for info also.

There are three things that control the on-off functioning of the compressor.

The first one is that blue snow flake switch that is the master on-off switch.

The second is a delay relay relay that might still be used that started back in the mid eighties.
I believe some over thinking engineer was convinced that the engine and its control system needed to get up to speed before the clutch should engage. The clutch will pull in or engage, stop and restart with the rotation of the ignition key when cranking.
This delay relay component may still be use or not? It could have been delegated inside the ECU later or dropped all together. You would have to study up on your year car (unknown) diagram to confirm either way. I have no experience in this as I have not taken the time to look for one on my newer '91.

The third item is a pressure switch built into a large accumulator can, that you see out under the hood. It is combination cooling, anti-frost up and low refrigerant cut-out, or completely off controller.

This last item will always show continuity or closed unless the system is almost empty or totally flat! unless, it is of course, has fail or is failing. A properly filled system at rest (not running) should show about 70 psi on average.
All you need is 20 to 25 to keep it closed. That is the lowest operating pressure or cut-out/off.
This switch IS the "decision brain" in the family of parts.

You have had a leak and you probably still do!!!
But you can still have just enough static pressure to close the switch but over time the change in outside temperature will lower the vapor pressures.
The expanding and contracting of the gases will trick you.

Some Latent liquid refrigerant molecules, clumps if you will, that get encapsulated (whipped per-say) into the compressor oil will keep boiling off and raising back up the pressures slowly.
Eventually, you will let out enough of the vapored liquid that it will stop playing with you mind!

Think of refrigerant as being like water but that it boils at very low temperatures.
If held under some higher pressure it takes more heat to boil it! Just like a home canning pressure cooker or our cars radiators. Under pressures, the liquid holds and moves more heat.

Hope this gives you an explanation for the weirdness and what has got things stopped! Beside getting some "pro-gauges/tools you could try the library for a book on basic refrigeration principles. It can teach you both home and automotive repairs at the same time, if the book in thick enough.

Phil

Did it stop working all of a sudden or are you trying to get it working because summer is approaching?
If it has no charge, it won't kick in.
Will it fire up jumpering the pressure switch?
You really need to hookup up proper a/c gauges to test the system.
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