Hey guys

I Bought a used Zacconi Riviera recently and after getting some new gaskets for the boiler/group and the PF I started using it. Right from the beginning the lever started jerking after passing parallel during extraction. I didn't change the piston gaskets and I can't say how old they are, but they did seem pretty decent, they still had a "soft" feel to them, unlike some of the other gaskets that were bone dry.
After attempts at dialing in the grind to see if that would help(it didn't) I took the group apart and cleaned it very thoroughly with Brasso. Before this they were some serious markings from what i presume was old coffee. The Brasso took most of it, but i can still feel the slightest of unevenness inside the group, which seems to be in the actual brass and not just a layer of coffee. After the cleaning i gave everything a good amount of Dow 111, both the piston gaskets and the lever pin. The action might be a bit more smooth now, but the jerking hasn't changed at all.

If i grind very coarse then there's little to no jerk in the lever, but that seems to be because the piston meets almost no resistance, the "espresso" starts flowing quickly through the PF in under 3 seconds of preinfusion and with a bottomless PF the espresso sprays everywhere when i let go of the lever so it can hardly be seen as a proper extraction. As soon as the spring and piston has to do any serious work, the jerking is significant.

I ordered some new piston gaskets, just to check if that was the problem, but I fear that it won't help. Does anyone have any tips for making the the action smoother? It might be pure speculation, but I kinda hear a something from the upper part of the group, like the lever pin grinding on something on the way down. I have tried both dow and lever pin oil from OE(not at the same time), and neither helped, so lubrication doesn't seem to be the problem.

Can't speak as to the effects of cylinder wear but I recently changed the piston seals in mine for the first time. The seals looked in excellent shape, well cared for, well lubed, but changing them out made the lever action extremely smooth again. I believe I learned that a very small change in seal diameter greatly affects lever action and is directly related to the amount of time that heat is applied to the seal(how long the machine is left on). Well worth trying new seals first before more involved solutions IMO. At least then you'll have base of comparison for how much the action improved.

Couple thoughts, on mine one of the seals faces up, so you have to carefully push in on the seal all the way around as you slowly compress the piston in the grouphead so the seal stays facing up. If you don't do this correctly, all or part of it will get turned upside down. Second thought, maybe the spring is possibly broken or not seated correctly in there. Third thought, if there is excessive wear on the lever where the pins go, maybe the piston is turned 180 degrees from where it was and causing issues now turned the other way. Fourth thought, maybe one of the pins was bent or otherwise damaged when putting back together. Final thought, the small pin on my mine has a metal bearing on it, do you still have the bearing? If missing maybe cause issues. Regardless, I would take it apart again, check everything for damage, and redo it.

Onion, I'm afraid none of those things are the problem. I don't quite understand what you mean with the lever turned 180, but there doesn't seem to be any kind of wear on the pins. Mcal I hope you are right, would be so much easier than something obscure.

It might be the seals, since I tried pushing it piston with seals into the group without the spring, just to check how much resistance there were. I could barely get the entire piston in, even when pushing like crazy and getting it out again was also really difficult.

Do you have the older 3 piston seal configuration? The middle u-cup seal's open end faces upward, so that you need to be careful when inserting the piston, as to not bend/damage the lip of that seal. It could also prevent smooth insertion and proper operation.
Recently I had a machine where 1 seal was just a bit different than the other two, obviously from a different production batch, and was causing similar issues to what you're experiencing. A new set of seals solved the problem. Compare your 2 to 3 seals and see if they're all the same.

This problem is an interesting example of the general physics idea that static friction is greater than dynamic friction. It is also an example of the fact that gases confined by a piston tend to act like a spring. Were there not trapped air under the piston the problem almost certainly would not be occurring. I have seen my spring lever do this with recently serviced piston gaskets. I have also seen it occur with ones in dire need of servicing! What is really happening is that the piston is "bouncing" because the trapped air is acting like the spring mentioned above. The liquid moves down, but static friction causes a slight delay in the motion of the piston. When the piston starts to move the dynamic friction is much lower, so the piston plunges downward, because the air is acting like a spring, and because the piston has mass, the piston continues to move downward until the pressure under it is greater than the spring's force, so it bounces upward. When it stops it is subjected to static friction again, but it now moves down quickly again, bouncing again and again. It is really an oscillator. The oscillation is enhanced by the momentary greater friction at the top and bottom of the bounching cycle when the piston motion stops.

Try this: Have your portafilter loaded and cup next to the fully warmed up machine. Press down the lever with neither the cup nor portafilter under the group, let a bit of water escape from the group, let the handle come up a bit to stop the flow, quickly put the portafilter on the machine with the handle nearly ready to release water. When in place push the handle all the way and let the space under the piston fill. Put the cup in place. Wait about ten seconds, let a few drops of espresso emerge, recock the handle. Whenever I have had this problem this seems to solve the problem because there is much less air under the piston to act as a "gas" spring. It also will increase shot volume. You do not feel a "spongy" pull with a spring lever machine!

Also a hotter group can reduce trapped air too because of the higher water vapour pressure.

The seals seem and feel identical and I got the middle seal into the group without folding the edge. It really sounds like a new set of seals will help.
rpavlis you seem to have done a great deal of research on the topic and i will definitely try it. One problem with your technique for me is that i can't get a preinfusion lasting close to 10 seconds. I'm using the OE slap shot and i really find it difficult to get this kind of resistance, at least if i want any kind of espresso after about 25 seconds. Any ideas?