I think opening the high side, even during a gravity fill is a bad idea. ac experts say to never open the high side.

 

So I went "old school" vacuumed down, closed the high side gauge, and put freon in the low side. Started the engine and brought it to operating temp and put the freon in and it is operating perfectly.
John

 

Final note: a vacuum pump does not and cannot remove air (atmosphere) from the ac system. A void exists only in outer space. The pump can remove moisture and create a negative pressure by compressing the air in the system, thus creating enough space in the system to accept the freon.
John