hardtailer

Take the cap off only when coolant/engine is at ambient temps.
Personally, I would drive it, get the temps up, drive it like you do on a canyon drive and shift at 6k (I.e. rev it!).
Drive home, lift the rear up high and rev it like that, car stationary and coolant cap tight ON the reservoir.

Revving it with the cap off makes it easier for the air to exit the system as the bubbles are bigger compared to when they are compressed at ~1.3 bar (or whatever the cap holds) but I wouldn't want to rev it past 3k till I see 150F coolant temp.
Putting a vacuum on would make the bubbles even bigger but if only half of the reservoir is air and the bottom half filled with coolant, there is just that half reservoir you can evacuate the air out of before you start sucking up liquid and as such the air bubbles in the whole system can expand only that much.

Ben8jam

Sorry all, coolant WAS decreasing when hot, but these last few times i guess it's actually been over the max line and just appeared to be empty. I must be getting old eyed

Mike Murphy

It can be hard to see fresh coolant. I’ve had that happen to me as well.