My Serve

This is my serve. I've been improving the technique of my serve the last few years. The second footage was recorded two years ago in Hong Kong. In that video I have an extremely bend of the spine after the trophy position that allowed me to hit with increased topspin but the chances of having back injury were greater. When comparing both videos I think there has been a significant improvement. The first footage my racket head is higher just as soon as the ball is released by my left hand, my back don't bend so much and for me it feels that I can hit the ball faster. Watching my videos in high speed video is great because it allows me to see what really is happening and what I need to change for my serve. In my case my number one technical aspect that I need to improve on my serve is by keeping the right leg and hip back through the execution of my serve. My right leg collapses after the maximum bend of the knees. Is it because of my hips that are rotating too soon? Is it because my right knee is a bit week?
I need more shoulder turn in my preparation and that is one aspect I will continue to work on.
Lately I've been trying to visualize Pete Sampras Serve. He has a totally different serve where he tosses the ball higher, has a great knee and shoulder turn. Perhaps the way that he serves will allow me to serve withou having my right leg collapse...