I assume that your good pressures were after adding freon. In that case your system is probably good. If pressures are good before adding freon you don't need any more. Adding too much will actually result in decreased AC performance and higher temps!! A good system will usually give 37-48F at the center AC outlet on the dash in 90F temps, colder when temps are lower. Pressure at the charging gage should be 37-45psi at 90F with the valve at the can CLOSED, this will give the actual system pressure. With the valve open you may get the can pressure instead. This has already been posted but is very important. Don't ever shake the can unless you are adding oil in which case read the directions. Shaking can introduce liquid into the system, bad! There are some good instructions posted here, shaking the can is not one of them. Make sure the engine is running at a fast idle, not slow idle. Only connect the can when the AC is running and at fast idle unless the system is completely empty. NEVER connect the can when the AC is not running unless the system is completely empty, especially when it is hot or at high pressure from running. These high pressures can blow out the can.