It's a primative Boeing thing, depressurize Landing Gear associated Hydraulics in flight, certainly 70,72,73,74 (classic), 75. I haven't reached the dizzy heights of 76 or 77 yet but suspect that the more modern ? have taken a leaf from the bus book and do it automatically. That would leave out the 76 then as that is really a 75 on steroids ! no idea about the 744 either but I expect it's similar to the 742.