I would make sure the shower screen isn't clogged. Also you might try the Robert Pavlis technique to make sure you are flushing out the air pocket before your pull. I can see that you might find it convenient to add the valve that reduces the air pocket when it becomes available.

Ah. Gotcha. I did misunderstand. If you are pulling turbos with your bright coffees then yeah go hot. The dry pumps will get you there reliably once you find the right temp.

The lever technique works to fill the group as full as possible. So long as you feel the lever want to rise you cleared the air. It can also cut down on puck erosion from lifting the lever. But yes with coarse grinds for turbos you will probably often get some bubbles no matter what. Sometimes it's channeling which you can fix but often it will do so no matter what so it's hard to lock down what's happening. Mostly I think it's that the coarser grind doesn't compact as much and can't provide the same resistance as fine grinds.

Just to clarify, this is normal for a coarse grind if you don't go through machinations:



For this roast, I can eliminate the cone of foam just by grinding finer. Grinding finer means that the wet puck will offer more resistance to the pressurized air at the end of a shot. The pressurized air quantity, etc., doesn't change; what changes is how fast the air is pressed through the used puck. Thus, this foam/bubble has no direct impact. However, grinding coarser/finer does change the taste of the shot - whether for better or worse depends on what you want to accomplish. This was a low pressure long ratio shot, so much coarser than, say, a ristretto.

I pulled a shot on my Caravel yesterday and saw that it had a bubble at the end, a clear one like first shown here. The shot tasted very good and the grind was too fine, if anything. I don't think the bubble is a "problem." Like the previous post, if it tastes really good you can let it be.