My former left hand grip was index and middle finger around the casing of the third valve and ring finger in the slide ring, pinky next to the ring finger.
I realized that in some difficult moments, I was squeezing the third casing:with my other trumpets I hadn't noticed any problem, but the Getzen valves of my new Lawler dislike that, and sometimes appears an issue at the upstroke, especially on the third valve of course.
I have now changed my grip: pinky under the third slide, middle finger in the ring. No finger doesn't squeeze the casings anymore. At the same time, I improved the position of the fingers of my right hand, to be sure to push the pistons vertically: issue fixed!
Maybe some of you have had similar experiences?