Forget about any thing your going to find at a tire store. Your dealing with 3000 psi bottle pressure and 750 PSI in the accumulator vessel.
What you need is an accumulator charge valve. $195 bucks, cheaper than a service call. Complete kit with hose and bottle adaptor here.
I’ve bought from Hydracheck before. They could do a M28 x 1.5 metric euro version way cheaper than any thing I could find down here. Good people to do business with.
Your playing with high pressures there, usually with out a pressure regulator fitted to the bottle. So you have to be careful.
You need to release the discharge from the bottle as slowly as possible. Two reasons, it only takes seconds to overcharge a small accumulator, and you need to charge the accumulator as slow as you can. The gas will expand and heat up in the accumulator, giving a false pressure reading. You want to charge it, then go make a coffee. Then check the pressure again when its cooled back down.
The other trick for young players is to ensure that the hydraulic fluid pressure is discharged, before you check the gas pressure.
If you only have a slow leak, you might think about totally discharging the gas and charging the valve and putting a new cap on.