ADDING FREON
Yes, it is sealed, but that doesn't mean it is leaking, otherwise why do you need to 'service' it every year? Small amounts escape under normal operating conditions until one day it doesn't cool so well. That said, you need to know what you are doing when handling freon and have some competency with the procedures. This can be learned, and there are lots of places to get guages nowadays. But you MUST be up to the task and it is best to have someone walk you through the first time. And of course read up on it and your guage manuals before you even think about checking your pressures. Get a real manifold guage & learn how to use it, not just those little guages that come with a can.
NOW, a '92 model Honda probably uses R-12. In that case you must convert to R-134-a or another equivalent. At least this means you must change the seals, dryer, expansion valve, etc with specific types of parts for the refrigerant you convert it to. Before you do this, I'm sure someone will pull down your old R-12 into their recycler for free. R-12 is expensive and they are not making anymore, so don't let anyone charge you for giving them a rare commodity if you even have just a little bit left in the system. Get a conversion kit, change the fittings & the seals, dryer, etc, -check with your supplier- then have it professionally vacuumed & charged. You'll still save a bunch by doing what you can & have a pro recharge it
One last thing, if it turns out you have a leak there is a probe guage you can get at Harbor Freight tools to check for leaking halogenated gases- ie Freon. Regular price is 69.99 but you can usually get a coupon or other bonus to sweeten the deal. This tool is better than the other way, which you can't do if you have R-12- no leak finder refill charge with red dye is available. The other way for 134-a is a UV dye that is added, or to light it with a special torch, but that can expose you to killer phosgene gas used in chemical warfare during WWI. Some things are better left to those with the equipment & expertise.
If you have 134-a, then invest in the guages (provided you are up to the task) and top off with a can with the UV dye, that way when you really have a leak you can find it with the special glasses & UV light it comes with.