Inflating air top suspension

I'm having trouble inflating my alko airtop suspension. I connect the pump to the Schrader valve and inflate to 2.5 bar, but when I try and unscrew the pump from the valve some air escapes and the pressure drops back down. As the air volume in the system is so small the slightest amount of air escaping drops the pressure consideably. Any ideas on how to disconnect without losing pressure.

Put a bit more air in than you need then you can let it out down to the pressure you want.

Hi
I recognise the problem happened to me many a time. I now keep a small 12v compressor in the tray under the driver's seat, bought from Aldi, which can be screwed onto the valve. With this type of connector, you can hold the connector and keep the inflation tube away from connecting until the very last turn. Likewise when taking off pull on the tube while unscrewing the connector.
Hope the above makes sense.

Hubert

I have found a solution from Amazon, 

[size=35]PRESSURE-RITE SCHRADER VALVE ADAPTER. [/size]