2004 Chevy Suburban A/C stooped working

To take the last question first, the system pressures are dependent on the ambient temperature and the humidity, the higher those are the higher the pressures will be. If there is a refrigerant leak, you might be able to see where it is by the oil residue at the site of the leak. Either way the system will best be served by purging the system and then evacuating it. The system capacity will be represented on an under hood sticker. The system pressures for high and low sides on an 80 degree day might be 225 high and 50 low. I always use an ultra-violet dye when recharging any system, it makes it easier to find leaks.