i have an old milk truck with an icecream freezer. its old and an r12 system. im going to recharge after evaucuting with an r12 replacement, r414a. the plate on the housing states charge with 12 lbs. what reading should i be seeing on the manifold gauges? im feeding it in thru the manifold gauges on a scale thru the suction side. thanks for the suggestions. the system uses cold plate evaporators.