Viking

There is ice buildup on the back of the refrigerator panel. There are two coils on this refrigerator only unit. A lower coil that supplies a fresh food bin and the main coil. It is just comprised of a few U bends soldered to copper strips. The refrigerant circuit goes through the lower evap first and then to the upper main evap. Both evaporators were iced up. There is no defrost system on this unit. It just uses timed off for defrost. I checked the thermistor and it tested fine. The lower fan and upper fan are operational. I deiced the unit and let it run for a week. The bottom evap was iced up again. The top evaporator had slight frost on the first two U bends of the coil. I'm refrigerator seems to be cooling just fine. I'm thinking it is low on refrigerant (slightly). I would think the frost would be further down the coil. What are your thoughts? Is there anything else that could be causing the problem? The unit only has 8oz of refrigerant, so I don't want to tap into the system until I'm absolutely certain it is a refrigerant problem.

anybody???