MrNev

The volume of compressed air in these mini expansion vessels is very low. When dis-connecting a foot pump type valve from the vessel you will loose considerable charge pressure as the process of dis-connection is relatively slow. Using a compressor with a traditional garage type push on connector ( the conector is just pushed onto the valve rather than locked on as in the case of a foot pump) means little pressure is lost from the vessel when dis-connecting. I have small ccompressor I used for car spraying probably about 4 cubic feet/minute with a gauge/filler but this is overkill. You can always dis-connect the vessel and take it to a garage. In reality 99% of mini vessels are flat. Providing there are no check valves upstream water will expand back into the main.