I have a CO2 cartridge pump. Problem with it is, once u press it to the valve the gas starts to exhaust and will continue to do so even if you remove the pressure. Is this the norm ? If so, can u buy a unit with some sort of trigger mech to start and stop gas release ? My reason for asking is this. Although I have not as yet needed to do so, if u do get a puncture and are replacing the tube while out on the road you cannot place a small amount of gas into the replacement tube to facilitate placement into the tyre. Am I missing a trick here ? Ta for sensible replies.