I've had Miss Pavoni (Millennium) pretty much dialed in now for a couple of years, but for some reason I can't seem to purge "false pressure" from the group anymore, resulting in a spongy pull that starts extraction sometimes 2/3 into the lever stroke. I thought I understood the physics of it, but I can't seem to get a routine going to achieve a good, solid, full stroke anymore...

Here's what I usually do: When the pstat kicks off during warm-up, I bleed off the trapped air while making a couple of quick pulls of the lever, using that water to warm my espresso cup(s). When I'm ready to pull a shot, I'll raise the lever until water discharges, then lower the lever just enough to cut the water flow, hold it there and insert my prepared portafilter. With the portafilter locked in, I gently raise the lever all the way to fill the group for about a ten second pre-infusion, opening up the steam valve for a few seconds at the same time.

You'd think that would purge the air in the group, but the lever is still soft for about 1/3 to 2/3 of its stroke. The pull is still usually about 25 - 35 seconds including pre infusion, and the espresso is good to excellent, sometimes awesome, with plenty of crema. I'm just puzzled why I can't seem to get a firm stroke all the way through these days...

Anything I'm missing here...?

Thanks!

leopolis wrote:When the pstat kicks off during warm-up, I bleed off the trapped air while making a couple of quick pulls of the lever, using that water to warm my espresso cup(s).

That should be perfectly adequate to eliminate the "false pressure" effect. You may need three or four of these to get your group up to temp though if your machine has just come up to pressure from a cold state. I don't think that spongy feel has anything to do with so-called "false pressure".

The spongyness is caused by air BELOW that upper seal. That air is gonna be there no matter how much preperatory pumping and bleeding you do. It is not driven out until the incoming water and steam in the brew chamber drives it out. On a pre-Millennium Pavoni especially, you can do mini-pumps of the lever at the top during pre-infusion, and this seems to help produce a solid feel, probably by working some of that air into the upper part of the group. I've not found this technique to work very well on my Millennium machine (Millennium has no vent at the top of the group, and my inlet hole seems to be right at the very top of the stroke.)

Things that seem to help me alleviate spongyness and increase volume on a millennium machine are:
1) Making sure the group is sufficiently hot. The hotter group should allow more flash vaporization as the superheated water enters the group, and this burst of vapor helps drive air down and out through the puck before it gets wet.
2) Dosing lower, with more headspace.
3) After a 5-10 seconds of lever-full-up preinfusion, press a little, then go back up for more water.

If this is something new, it might be that your water delivery has slowed significantly. Either due to scale in the water path or due to the piston starting to come unscrewed. Another possibility is that you changed to a less dense bean and have less puck headspace than you used to.

Thanks, Pat, you bring up some points I hadn't thought of. As it happens, my efforts to avoid overheating might be part of the problem, per your suggestion. I'll also pay attention to how things change with dosing---I am dosing pretty full these days. Thanks again...