A year on and the pulls are often bouncing or spongy. I notice it when releasing the grip on the lever as the shot has been completed.

A concern or is it just the nature of the lever beast?

The details...
Time to lift the lever, about 13 s.
Time to pull the shot about 30 s.
For 15g in and 28g out. Always dark roast.
Opened and cleaned the piston with a titch of lube. Smooth up and down strokes. Didn't take the piston apart but something rattles in there. The white sealing ring likely.

Espresso tastes just fine. Yes!

If I shake the lever up and down a bit at the end of the upstroke, I'll get some air bubbles. But not always.

That's it,

Dave

Probably worth taking the piston apart to see if the small gasket is sound.

I would give the small gasket a life expectancy of no more than a year. I use mine rather a lot, it is my go to home machine and I would say around 4-10 cups a day are going through it. Almost 10 years old now.
Granted, I am using the older ES3 version, maybe it is harder on the small gasket. Don't think there is a big difference though.

You can order spares from Wouter, or if you know the dimensions you could try some local gasket supplier.

I have the ES3 but use much less frequently, taking breaks of weeks or more and daily max 2 shots.. The middle seal does not last more than a year for me either. Perhaps less. The newer seals from Strietman have better feel but lifespan seems to be less....I don't mind too much.

I still love the machine.

I'm only on my 2nd middle seal after 4 years and around 4000 shots and it's working fine. Spongy pulls are the piston not opening right away so you pull air in the group instead of water, thus a spongy feel.

Raising the lever for 13s is pretty long (especially for darker roasts) and if you do it very slowly the small force can keep the piston closed. This happens to me mostly after lubing as the piston moves up so easily on the piston seals. You're always pushing down hard on the center piston seal so the middle kinds of can get stuck in there which is pretty normal. Just pull it up with a thug the first cm and then slowly raise it to be sure you opened up the piston and all of your problems will probably disappear.

I'd second this comment on a 13s lift being the potential culprit. I do 6s and find that if I go much slower I also tip over into the spongy zone. Gotta make sure that inner piston is moving. Fingers crossed a little change in technique gets you moving in the right direction!