I've fitted
square section o-rings to mine and a
Schrader valve. I half fill with water and pump them up with a car tyre pump for testing. Then I check the pressure over a day or two. It drops initially because the tyre pump heats the air but the pressure should be steady after 12 hours. This also enables me to see at what pressure the relief valve 'lifts' at. I put food-grade grease on the rubber seal and threads. I tighten by hand until the barrel starts to turn.
I got fed up of the 8g CO2 bulbs lifting the relief valve and wasting half the gas, so I now use 16g bulbs and a bicycle tyre inflator - with this you can add puffs of gas and stop if the relief lifts.
Something like this