Finn
I had a RX7 engine with two A/C cores for radiators, but here is what I found worked
1. Fill as best as possible, then use aircompressor to take it to operating pressure while cold
2. Run engine up to operating temperature, using highest revs possible on ground when warm
3. While hot, bleed air pressure from system, when cool, replenish reservoir
4. Repeat at least once, then go fly, and check after flight to replenish.