I have a 79 IMP with 228 Mercruiser. The trim runs up and down. There is fluid in the trim reservoir. My problem is when I shift to reverse the thrust lifts the sterndrive and when I shift to neutral, it clucks back down. On the trailer, I can shift the engine into FWD or RVS and I can lift the motor, but I can't push it down past the set point. I filled the reservoir and ran unit up and down many times without improvement. Do I need to do a full bleed or is there some other part I need to address.