no overdrive 4r75w

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DESCRIBE overdrive
My OD went out in my f150. When I down shift to d2 the brake barley slows the truck speed. When I opened the pan the snap ring had come loose and slid down but not out. I reinserted a new snap ring but there is no change. I have used a setup that will allow me to put pressure on the of servo and I watched to see if the brake band is tightening. When I apply pressure to the servo the fluid leaks out of the bore and the band starts to tighten but falls back as fluid leaks out. Is this normal or do I need to install a new servo? Also when the OD went out there were no warnings of any pending failure.