We seem to have an air leak when the back edge clamp is opening, eventually it reaches full pressure and the clamp will open up and clamp the plate.
Any idea?
The incense stick method is usually the most effective way. Light an incense stick and keep it close to any areas where you suspect an air leak (look at the typical places for air leaks below). The smoke will be sucked in at the places where there is in fact leakage.