This seems to happen regularly and just dismantling and reassembling the 'group' has always solved the problem in the past. But this time, it hasn't done the trick.
Before the problem starts, there is a small 'judder' when you lift the lever to the top and the lever goes 'firm' with drops of coffee starting to fall. Later in the cycle, the lever needs to be moved up and down near the top and, eventually, water seems to get into the group and you can squeeze out some coffee. In the end, the lever gets 'heavy' ,indicating that there is pressure in the group but no water. It just struck me that the feed spout is not operating and steam must be getting into the group, rather than water.
Has anyone else had this problem?
Andrew

Which generation of machine is this? If it be a late 2nd generation machine or early third generation one you have one with the dread polyphenylsulphide piston. They tend to come unscrewed from the piston holding rod. The inlet tube normally does not become blocked, though I suppose it could with really terrible water. The design of first and third generation machines is also very different from the second generation ones. (First generation: 1961-1974, second generation: 1975-2000, third 2001 to date. I believe this is right?)

The plastic pistons seem to have been mostly in machines made between 1997 and about 2006 or so, as I remember. Someone may have exact dates. The solution to the plastic piston problem is to get a replacement piston.

Thanks for the reply. The piston seems well fixed on the shaft and the inlet tube is clear. I have checked the holes in the upper and lower filters and they are free. I have checked that the feed tube is free and I don't think there is any leak round the tube as it goes through the black plug. That would account for pressure but no water so I tried to check by blowing and finger blocking in both directions. The tube is a bit corroded but hardly enough to have a leak in it. I guess I could try replacing it and the black plug.
I have been suspecting the vertical range of the piston relative to the hole in the plastic sleeve but that seems OK (and hardly likely to be changing as everything is fixed on the shaft. Whenever I look at the works of the Pavoni, I am put in mind of my old 2 Stroke motorcycle engine. They are the same in that, with so few moving parts there is 'nothing to go wrong' haha.

You mentioned that the piston is fixed on the piston rod - have you removed the piston entirely and ensured that the piston face is screwed completely tight against the rod? One time when I serviced the group on my 2nd generation Europiccola, I did something that allowed the piston to unscrew ever so slightly from the piston rod, and the result was as you describe - water flow was extremely restricted and pulling the shot was very difficult. I finally figured out what was happening when the piston unscrewed to the point that I couldn't lock in the PF and the shower screen popped off! Once I got the piston firmly threaded all the way on the rod, the issue was resolved and never returned.

Andrew, you neglected to directly answer Robert's very important question about what generation machine you have. But you did mention the black plastic plug and the plastic sleeve, so we all know that it is a 'generation 3' with the millennium group.

The likeliest cause would be an unscrewed piston, which is very common with the plastic pistons, but looks like that has already been eliminated in your case.

A hole or connection leak in the siphon tube and plug might explain it, but unlikely, and I think you checked that. (You can remove the group with the piston still in it and blow on that tube to check.)

Another somewhat unlikely possibility, but one that has been seen before is that there is bead of scale blocking the top of the inlet channel of the plastic sleeve. This would allow the pressure to build up in the group, but would prevent a good water flow when the lever is up. Was discussed once in a different forum here.

P.S.

sophiecentaur wrote: I have checked the holes in the upper and lower filters and they are free.

I can't figure out what this sentence is about.

I really can't say whether it is the piston or not, but it sure sounds like it could be. I just wanted to add that the plastic piston is known to loosen, but that the brass ones can do that too. In fact, I never experienced the problem until I replaced my plastic piston with a brass one. As it is now I have to tighten the piston rod into the piston about a half turn every couple of weeks when I lube the piston seals. It doesn't take any time to do it, so I don't mind. The next time I pull the piston out completely I will put some Teflon tape on the threads to see if it won't tighten up the fit a little.

Well dang mah hide!!!
I just looked at that piston again and I see it has come unscrewed (held quite firm but it's about 10mm down its thread). That explained pretty well everything. I must look into a method of locking the piston onto the thread - unless someone suggests a way to replace it. A tiny pin through the shaft could do it.
Many thanks for this info, guys. These new fangled Forums really do make life easier for the DIY inclined.

The reason that the polymer pistons come unscrewed has to do with basic polymer physical properties. The glass temperature of polyphenylsulphide is about 83C. Above that temperature the material begins to soften more and more. EXTREME pressure is placed on these threads, and this slightly deforms them. Over time the "creep" continues. Polymers are often described as viscoelastic, because they exhibit properties of viscous liquids and solids.

La Pavoni eventually abandoned the plastic pistons and returned to brass. The brass ones are interchangeable with the junk plastic ones. It is not too hard to machine one from brass, by the way. Another thing happens with crystalline polymers like this is that the thermal cycling has a tendency to increase the degree of crystallinity. This results in shrinkage, and it also can weaken such materials, so they fail. Ordinarily brass pistons seem not have this problem.

sophiecentaur wrote:I must look into a method of locking the piston onto the thread

jtrops wrote: The next time I pull the piston out completely I will put some Teflon tape on the threads to see if it won't tighten up the fit a little.

I think if you get the threads really clean and grease free, then put a Phillips screwdriver shaft through the hole in the piston rod, then grasp the piston in your strongest hand and hand torque as tight as possible it might stay put. As jtprops points out you can check it fairly easily each time you service the group. If that doesn't do it you might resort to a food safe threadlocker - see La Pavoni piston thread-lock

I don't think the teflon tape trick will help, as it tends to lubricate the threads. No harm in trying that though.

Andrew - you didn't say whether your piston is brass or plastic. If it's plastic I would suggest budgeting about $50 US toward the purchase of a brass replacement.

I had a brass piston in mine and once I really tightened it down - and I'm only talking as tight as I could get it using my hands - it never moved again, once I had identified the problem.