Don't take this the wrong way......but
Rich is the only person here that is giving any advise worth following.
I can't believe what I'm reading here, from people that are pretty knowlegable in other areas.
Gauges are the ONLY way to accurately diagnose your a/c system PERIOD!
Even with gauges you must be familiar with what they are telling you.
High and Low side readings are only a small part of the equation. They can fluctuate, the needles may bounce, they may stay steady or rise and fall on one side only, etc. indicating different problems.
If you have a "significant" leak FIX IT, it is pointless and irresponsible to keep feeding your a/c system refrigerant.
Adding oil (if not needed) is one of the best ways to decrease your a/c's performance.
You NEVER add refrigerant to the high side (unless you have evacuated the system, pulled a full vacuum on it, and with engine off....are rapid charging, and then fine tuning with the low side).
What are pressures supposed to be? They change with the ambient temperature (measured a few inches in front of the condensor). So, unless you can match the amb. temp. stated in the FSM then you need to know what temps. correspond.
As far as a sight glass is concerned... the Cruiser is sold in quite a few countries where their primary use is far away from a qualified a/c shop. It can be used (remedially) to check for certain conditions...with the understanding that you also know what lines should be cool or hot etc...
Too much refrigerant= poor cooling, high head pressures, and worse.
Too little = poor cooling, clutch won't cycle, accelerated compressor wear...because the "freon" carries the oil", etc..
Too much oil= poor cooling in a hurry.
Can you educate yourself, obtain the PROPER tools, and service your own vehicles? YOU BET!
But a little can of oil and freon, with a .50 cent gauge on it is a sure recipe for disaster. I don't care how many people have "thrown" a can of freon into their system and made it cooler, it will bite you in the butt eventually.
One trip to an a/c shop to replace a compressor, drier device, flush (if not replace compents in your system) pull a vacuum and charge your system.....will cost you half again what you could purchase gauges, flush canister, and a good vacuum pump for.
For those wishing to do their own a/c work....get the equipment, and study.
I've been doing all of mine for 20 yrs. and have only had to add 134a gauges to my other tools. I have saved thousands of dollars over the years, and I don't have to wait for my vehicle to be serviced on their schedule.
If you're not comfortable with learning to do your own, or you suspect a potentially serious problem....take it to a professional.
Leaks......your local a/c professional will love you for putting that "leak stop" in there, more money for him.