The Sonnax fix requires a new oversized valve and a reamer to enlarge the bore. Most rebuilders just do what the YouTube video shows. Sometimes they use the long spring or sometimes they use a piece of tubing or anything between the two PWM/TCC valves. It's an easy fix that essentially stops the PWM (pulse width management) valve from working. The valve is constantly working back and forth so quickly that it's almost a vibration. That's why the bore wears out. GM likes their Torque converter clutches to have a controlled slip for smoother apply and operation. This creates problems down the road. Any transmission rebuilder with any sense will block the valve with any rebuild. Transgo has a slightly oversized valve and a stiff spring in their shift kits and they work just a good as Sonnax at a fraction of the cost. You will barely notice the fix except for a slightly firmer 3-4 shift. I did the Transgo kit to mine and I love the firmer 3-4 shift.