Ive got a small refregeration unit that holds 17 ounces of r12 that im dropping mp39 in to. Its one of those 2 door type units..2 evaps...its basicly a prep table with 2 doors below it. This unit doesnt have a high side access. I dont know why they dont put access on these little suckers. The origional call was the evaps freezing up..i suspected low on freon even though im having great difficulty finding the leak..its very minute..i got a slight reading with my detector but even with soap bubbles have not located it. Anyway I weighed in 12 ounces of mp39 before it started going off on the thermal on the compressor. Should i just go back in and weigh in 10 ounces or less...i know mp39 runs at higher head. Hes only using this thing another 3 months..i guess i can put a saddle valve on the high side although i dont like them. I cant stand drop ins with the different pressures..i dont consider that a drop in.