It's 2014 and I don't see any replies. I just bought a '91 Chris-Craft with a 574APRPWS 5.7 and an OMC Cobra outdrive. The power trim bled down. The standard troubleshooting routine for that is run the outdrive all the way up, then lower it a few inches. Disconnect the hydraulic lines at the top of the cylinders (furthest from the boat), putting a catch pan under each cylinder. Look for continued weeping of fluid after the initial amount when you disconect the lines. Whichever one weeps needs to be rebuilt.
I looked at cost for rebuild (about $38 for a seal kit for each cylinder, don't just rebuild one) and saw I could get newer, used cylinders for about $150 and went with the new cylinders. I found some oil on the old port side cylinder rod, so I decided to replace them both. I did not use the routine above or do the rebuild for the sake of time. I put the new cylinders on and I'm now in the process of bleeding out the air.
Bleeding air out. Raise the outdrive. I'm using a hand, pistol grip oil can to fill the reservoir until the fluid starts weeping out of the hole. Replace the plug. Run the drive down and up a couple of times, stopping at up. Wait about 30 minutes before raising the drive, if it has dropped some. Remove the plug from the reservoir, and fill again. Repeat until the drive stays up overnight. NOTE: If you pop the plug (screw out actually) right after running the drive down and up, you will get a fluid shower since the air pressurizes the oil as it rises to the reservoir. I've had my showers.
Final note. I'm using Dexron II-equivalent ATF for the trim system. It's the same I use in Mercruiser systems.
