I recommend not using pressure, but letting gravity work. To do this, I use the bags from pearl.ch ("foldable drinking bottle for sport & leisure"). The 2-litre bag builds up quite a bit of pressure due to its size (unfiltered water), with the 1-litre bag (possibly no longer available) I collect the filtered water and then fill it into bottles. The 2 litres are filtered in 10-15 minutes. Can be done during a break when making a coffee. When the flow rate is low: After one or two minutes, push the air backwards out of the filter, then it will flow much more. By the way, the pressure can be increased by using a hose and holding the filter at the bottom of the hose: higher water column = higher pressure.