If you have very high pressure on the high side, which is usually all the little refill gauges read, you almost certainly have a stuck H valve, or metering valve. Typically it is bolted to the firewall where the pressure hose bolts to the firewall. There is a square block that the hose bolts to. They typically get ashed up on the valve and stop working. I have replaced 5 over the years and they act like they are low on freon, fog coming from the vents, but when you measure the pressure, it is very high on the pressure side and not much on the suction side, since nothing is coming through. You can replace it yourself. Get new ones, if you have someone that can salvage the freon (suck it out) that is a nice thing, they can even reinstall it when it is fixed if they are set up right. But if you are in the back yard, you can release it with the schrader valve, do it slowly so oil does not go everywhere. Once the pressure is off, unbolt and tape over the pipes to keep moisture from getting in. Replace the block, using new seals, aluminum, and vacuum out the system. If you do not leave the system open and replace the block quickly, you should not have to replace the drier, but it is recommended any time you open the system. Note if significant amounts of oil spill and replace about what you think got out. Pull a vacuum (or better yet pressurize it with nitrogen, see if it holds or if the pressure changes overnight.) Some manuals warn against leaving vacuum on the system for extended periods. If it holds, refill the system and you are good to go. Do not dump liquid refrigerant in the suction side, let it boil off as a gas can rightside up, put in warm (not hot) water, or hose down, if needed. If you do the best way, you will need a lot of equipment. If you do it yourself, you will have to go somewhere to get a vacuum pulled on the system. If you have a compressor, you can get a cheap vacuum generator that hooks to an air line (if you have the cfm). Happy AC fixins.