Also check the Schrader valve at the high and low pressure ports for a leak. This is easily done with the old spit test like we used on bicycle tires. If the valve leaks it will cause the spit (or soap film if you prefer) to bubble. If a leak is present, tighten the valve stem with the appropriate tool. If still leaking, the valve stem needs to be replaced...which can be dangerous to do given the pressure in the system. You may want your technician to do this for you.
Additionally, there are more "O" rings to consider. A refill with R-134a containing fluorescent dye can help you pinpoint the leak...especially if you use a strong UV light and the special glasses.