Very good observation.. although this isn't the same machine, this check valve does exist in mine as well. You got one bit wrong, though. The water doesn't flow around the threads of the part. It's actually a hollow bolt in which sits a spring and a little rubber ball pushed up against the boiler outlet. The dispersion screen is held against the bottom of the group via the bolt. It is slightly convex downward, so there is room for water to flow freely from above the check valve to be evenly distributed over the area of the dispersion screen. The purpose of the check valve is simply to prevent spent coffee soup from entering the boiler after the pump is switched off - basically doing what a 3-way valve does except for releasing the pressure. The valve opens at about 6 bar, tested by simply running the pump without the pf locked in. The way I understand it, since the coffee puck presents a higher resistance than this check valve, the pressure in the system is thus limited by the main relief valve I installed. I hope this makes sense.
edit: Here's a pic of the screen, bolt, and spring removed. I figured that would explain it better:
by oofnik on Sun Dec 17, 2006 8:32 pm 
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