Volvo Penta 290 Trim Help
 

I have an AQ151 with volvo 290 outdrive. When I put the outdrive up it will gradually fall over a couple of days. It's the older style trim pump without the external holding tank. It's a 1989 Imperial. I have purchased a newer style trim pump which I hope will fix the problem. My question is do I have to bleed the system or will it bleed itself by going up or down and if so how do I bleed it. Also, is my diagnosis correct? any help is appreciated.