It looks similar to my Bianca. The first two flushes release ample amount of water then reduced after that so I remove the portafilter like you. I remember noticing this after installing the Matrix shower. However, I don't really give any more attention to it. Just release the pressure every 4 flushes.

DaveC wrote:how old is the machine?
How often do you backflush with detergent?
Do you remove the lever and lubricate after backflushing with detergent?
Have you ever removed the lever and lubricated the camp pins etc.?
What do you lubricate them with?
How often do you backflush with detergent?

The squeak is after you lower the lever, so it's not the cam pins squeaking. More water should come out than that when you release pressure, or backflushing wouldn't work.

Remove the lever, take a detailed photo of the cam pins....I suspect the vent valve isn't opening properly sometimes.

Thanks for your reply, Dave. I have the same suspicion - something is preventing the valve from opening, but I can't figure out why it doesn't happen every time. I think the wailing sound may have been my kids in the background. I backflush daily and backflush with detergent every few weeks. The machine is just shy of 18 months old (this has been happening since new). Yes, I have removed the lever and lubricares the cam pin (lubricated with Dow 111) - I tried everything I could think of before I contacted Clive for help. Unfortunately, they didn't offer any help.
Here are a few photos






EDIT: Here are a few photos of the vent valve and path.

Preinfusion valve cam follower pin looks fine...you could try (very carefully) unscrewing it 0.5 to 1 turn. You have to be careful because for some unknown reason manufacturers of the groups like to threadlock these pieces and the thread can snap!! I don't know why they do this because changing the viton seal would cost a few cents as would changing the cam follower, and be easy, instead you have to buy the whole valve and preferably the OEM one. No thread lock means you can also "fine tune" the opening if necessary.

Don't backflush with cleaner every 2 weeks, not necessary for domestic use...do it every 2 months. However...do remove the shower screen, clean it and the dispersion disk/area thoroughly every 4-7 days. you will notice a big improvement in taste.

P.S. You can grease the triangular stem bit as well...won't hurt.