bidoowee wrote:Hi Dominico,
I read this thread when we discussed it last week but it's taken me a while to get round to writing a reply. I'm curious about where the air in the chamber is coming from. Obviously there is a small amount above the puck and shower screen when the portafilter is locked in (considerably more when using an empty portafilter such as in your setup). When the lever is lowered it creates a negative pressure in the chamber which could draw some air through the puck (or in this case, the valve). The other possible source would be if the water for your group comes from the boiler (which I presume is not the case) and the negative pressure is great enough to draw some air/steam along with the brew water. This seems pretty unlikely, and couldn't happen with a machine that gets brew water directly from the line as opposed to the boiler. So I was wondering what would happen to the "catch point" if you put a check valve in the line somewhere and then pulled two shots in a row. The first shot will guarantee that there is no residual air for the second. You could also just simulate the check valve by closing your valve just before the end of the flow.
As it so happens my groups are fed essentially from boiler water: they are fed from the thermosiphon which also regulates the group temperature. However I believe that air from that is minimal if any. I think the large majority of the air is coming from what you described: the air between the coffee bed and the piston at lock in plus the air pulled up through the coffee as the piston is lifted.
I did notice that the lever catches lower if I let water run through for a longer period of time before raising the lever: I attributed this to allowing more air to purge from the group.