Sorry if this is an elementary question, but I've read around, looked at Hauck's CAD videos, etc, and there's conflicting information-- I'm still unclear...

Hauck and others describe that hot water (and steam if in a gen 2) flow from the boiler into the space above the piston when the arm is down. Where does that water go when the arm is raised? Some seem to say that flows down channels to fill the space below the as the arm is raised, and THAT water is forced through the puck to become coffee when the piston is lowered. This doesn't make sense to me, because the pressure created by lowering the piston would force the water back up those channels in addition to pushing it through the puck. Right?

I've also read that when the arm is raised, the piston simply uncovers the opening of the pipe from the boiler allowing hot water to flow from the boiler into the space below the now raised piston, then is pressed by the lowering piston to become coffee.

I don't understand the water (and steam in gen 2) existing above the piston when the arm is down, and where it goes when the arm is raised.

Again, sorry, I suspect its an obvious answer to many here.

This doesn't make sense to me, because the pressure created by lowering the piston would force the water back up those channels in addition to pushing it through the puck. Right?

Here's how I understand it:

As the lever and piston are raised the piston seals travel above the port, uncovering it and allowing water to flow from where it is temporarily stored (above and around the brew chamber) into the lower brew chamber itself. (If flows under pressure from the boiler).

As the piston is lowered the seals cover the port again. This isolates the water in the brew chamber and prevents it from being forced back into the temporary storage area above and surrounding the brew chamber. The only place for water in the brew chamber to go now is through the bed of coffee. The piston downward creates the pressure to force it through.

Mark_in_NYC wrote:Where does that water go when the arm is raised?

If you look at John Hauck's excellent model of the Gen2, note that there is a vent back to the steam boiler at the top of the group as well as an inlet hole further down. When raising the piston, the upper seal flares tight, and water above that seal flows back to the boiler. Initially some of it flows back via the the inlet hole and down the dipper tube, but after the upper seal passes above that hole it all goes back via that upper vent into the neck of the group, which is open to the boiler on a Gen2.

When the piston is full up (as in the pic below) water enters the brew chamber below the piston due to the pressure in the boiler. When you pull the lever down, the lower seal flares tight, passes the inlet hole, and water is forced down through the puck.


Image courtesy of John Hauck: http://www.youtube.com/watch?v=qQaqW0SAJjs

and water above that seal flows back to the boiler. Initially some of it flows back via the the inlet hole and down the dipper tube, but after the upper seal passes above that hole it all goes back via that upper vent into the neck of the group

That's what I didn't get. Thanks!