daffy, a slight drop in pressure is perfectly normal after filling 5-10 bar is fine. while filling the cylinder gets hot and tge compressed air expands, then as it cools it drops in pressure.
I once sent 1 of mine to get to get the power tweaked becauae it was running about 10 ft lbs. I took it to my local shop who sent away and they had it 3 months. they said they had a flood and everything had to be moved upstairs. after 2 months I phoned and was quite shitty with them. I demanded to know what was happening and what stage my gun repair was at. the guy on the phone went to look and see,