I have a 1989 Ranger Bass boat with a 150 Black Max Motor OC197746 Model number 115453GD. The Power tilt on this boat raises and lowers as it should without any problem. However, the motor will not stay in the up position. It immediately starts to sink slowly down until it is in the full lowered position. I have replaced the fluid in the system and bled all air. I see no fluid leaks and the control knob on the pump is fully closed. Any suggestions on what to look for or test that would identify the cause? 