trigger814

Hello all, I'm a newbie to the forum and looking for alittle advise. I have a 1987 mariner 45hp that will not hold a trimmed postion. I can't find any leaks in the system, the fluid level is good, but when i bleed the system it works for a couple of days and then right back to dropping from full up to full down in about 20 minutes? I'm thinking that it may have dirt sticking the valves open or i may have to replace the valve body, i think? has anyone had anything similar and how would i go about flushing the system? thanks