I have personally serviced my La Pavoni Stradivari for the very first time (seals etc) and all has gone well except for one detail.
Previously (these last 10 years), I have been able to lift the lever to the uppermost position, wait until a few drops fell then proceeded to pull a shot. This time, when I raise the lever, it does not stay there, but finds it way to the down position automatically.

Can anyone suggest what I may have done incorrectly during servicing, or not done but should have, so that I may return to previous expectations/habit?

My thanks
regards
sean

Do you hear the hiss of water when you lift the lever at the top of its travel?

I would suggest checking the orientation of the seals at the piston. The flare of the top seal should point up, while that of the lower seal should point down.

Yes water emerges at lever top position.
Piston seals correctly placed.

Lever now stays up, until I bring it down manually.
I reduced the bean weight from 16.5g to 15g.