My machine is an eight-year-old Europiccola, and an odd thing happens when I make a second cup. Just as I'm nearing the end of the pull, bubbles and froth start coming out of the portafilter, almost as if trapped air is the last thing to come out. There's a bit of a gurgling noise. Is this a bad thing? Why does this happen, and how can I prevent it?

Sounds as if by the time you are doing the second pull the grouphead has heated up and hot air is forcing its way through the puck from the additional pressure above. Is the second pull noticeably more bitter/sour than the first pull? What you might try doing is cooling the grouphead before the second pull.
Here is a discussion of a few methods that people use to cool the grouphead when it has heated up... http://www.home-barista.com/lever...d-towel-t9478.html ... Try one of them to see if it makes a difference.

Does hot water drip from the grouphead when the machine is hot but the handle has not been raised? This would mean that water is getting past the piston seals and need to be replaced.

No water drips from the grouphead. In fact, I just replaced the gaskets, so there is no problem of leakage.
I haven't noticed more bitterness in the second shot--I'll do another taste test.
The bubbles don't seem to happen every time. Maybe I'm not holding the lever up long enough to let the grouphead fill completely with water. Nevertheless, I'll try cooling the grouphead as well.

MarcusAntonius wrote:No water drips from the grouphead. In fact, I just replaced the gaskets, so there is no problem of leakage.
I haven't noticed more bitterness in the second shot--I'll do another taste test.
The bubbles don't seem to happen every time. Maybe I'm not holding the lever up long enough to let the grouphead fill completely with water. Nevertheless, I'll try cooling the grouphead as well.

Every shot or every second shot?
Yes, I usually leave the lever up for at least 20 seconds to allow for some infusion through the puck. Perhaps that is it... You just need to allow for the grouphead to fill a bit longer on the infusion during the second shot when the grouphead is very warm. I would also try cooling the grouphead... Can't hurt.

Well, I usually only do two shots in sequence (morning cappuccino for the wife and for me).
I tried a longer infusion--even tried the Fellini pull. Same result.
Could it be that the portafilter has become too hot and is boiling the coffee as it drips out? I'll try cooling it before the second shot tomorrow morning and report back.

I had the problem of bubbles... instead of the constant "mouse tails" of dense crema I got increasing bubbles like boiling froth. The cure is to grind finer, tamp harder or use the single basket. Any of the above should be worked on until perfection is achieved. The single basket takes a coarser grind. Forget about fellini multiple pulls... the first 20 secs deliver the best coffee, never run past the blonding part of the run, just add fresh hot water. Grind... you just have to spend the money and get a good burr grinder. Small increments in grind are essential several days after opening the beans and they absorb moisture, what works first day tends to block the next days pour so the grind needs to get a bit coarser. And the tamp... first essential is to spread the grind in the basket without any pressure. A good doser will slightly overfill so do not push down with a finger, instead run a finger around the rim touching the rim mid joint with the fingertip elevated slightly hovering around the middle of the basket. Barristas make a vee with two fingers, scoop around to make a wee pile of spare ground in the middle which gets wiped off the top. No tapping and no holes, a nice even distribution. Next, rest the tamper square on top. Just the weight of the tamper. Remove and clean off the ears and neck. Now tamp firmly, as though you were using your weaker hand, and as you lift off give a final polishing twist. Check you have a perfect, level puck surface. If the lever is real hard to pull thats good, tho you dont want longer than a 21 second pull. If so, notch up the grind and try again. Similarly less than 21 you should grind a notch finer. And with the pavoni, wait with the lever arm up for 10 sec (or until the coffee starts to drip) before the 21sec pull begins. This pre-infusion is better than fellini (additional) pulls.

I've solved the problem. Looks like the porta-filter was getting overheated between shots. It sits in the group head while I steam the milk for the first shot and gets very hot--hot enough to boil the second shot as it comes out. Now I run cold water over the porta-filter before inserting the second basket. Works like a charm. (I'll work on the grind too, though, just in case!)