Cobra trim/tilt loosing pressure

92 Four Winns 4.3 Cobra.
Last year I noticed the outdrive would loose some trim extension as it sat on the trailer over the weekend. Checked fluid level and external leaks but this does not seem to be the problem, the bypass is snugged too. I have the drive pulled to reseal the lower prop shaft and replace a torn exhaust bellow (going to use an OEM part this time) and thought about diagnosing this trim issue also. I have a local hydraulic shop that could test the components if I bring them in, but is there some on boat diagnosis I can do first, to pinpoint which component is failing? The motor spins fine, the rams extend and contract and I don't have a problem with it in day to day use, but I know it will get worse.

Can I place long pipe clamps on the rams and apply some pressure to see if one collapses more easily? If I just compress one ram now the other extends, so that doesn't pinpoint anything. I purchased a couple of used rams on ebay over the winter (cheap) and had them rebuilt, but I don't like throwing parts after a problem.

Thanks

I forget what the lines look like on a Cobra, but there is a generic way you can test for cylinders bypassing internally on anything where the cylinder extends to lift (from a tractor loader to an excavator).
1. extend the cylinders with the drive on
2. support the weight of the drive
3. disconnect each "rod end" hydraulic line
4. leave all fittings open to atmosphere
5. remove the support to let the base end trapped oil support the weight
6. wait for it to drift down

whichever cylinder has oil coming out the open fitting as it drifts, has a bad internal seal

if they both leak out, they're both bad

if neither leak while it drifts, the issue isn't in the cylinders

Would you test 1 ram at a time and check for leakage, or attach both at once?

the way I described tests both at the same time, and you don't "attach" them, because we never detached them - just 2 lines to disconnect..

I've been a heavy equipment tech specializing in hydraulics and teaching it for 25 years - I use that method on anything that extends to lift. simple and easy.
detaching and pressurizing cylinders is dangerous and rarely leads to any proper diagnosis... don't make up your own process...there's a way you can very easily split both barrels wide open so don't mess around with that. let it drift with the weight of the drive and gravity is the only power source needed - nice and safe.

maybe my step 4 should say "leave the two lines and ports open to atmosphere"
should look like this

then kick the wood or whatever the drive is sitting on out from under the drive or lower the tongue of the trailer and wait for the drifting

also - cobra cylinders have relief valves right in the pistons, so if you do determine 1 cylinder is bypassing, it may not always be a seal- the reliefs can leak oil through the piston

Wonderful information, as soon as the bellows arrive I'll bolt it back together and give it a try. As I look at the rams, there appear to be two lines going to each one, but I will only remove the one shown in the diagram. If the drive doesn't drop overnight then I'll assume something is amiss in the valve body and take it to a local hydraulic shop for inspection

it'll drop - the indicator is where oil comes out. or doesn't

Now I understand. I thought the drive would stay up if the rams were good, but as I understand you now, the drive will drop even if the rams are good. It is just the location of the leak to determine where the problem lies.

Thanks again.

Parts came, resealed the prop shaft and it held 15lbs of pressure for 24 hours, replaced the exhaust bellows too. Next step was to figure out the drooping trim/tilt, but to my surprise it has been holding position for several days. I had previously checked to make sure the bypass was snug, but in needing to work on the outdrive had loosened the screw several turns, move the drive up and down manually and then resnugged it. Perhaps this movement caused the seals to seat better?? Anyway, if this happens to you, don't just check if the screw is tight, back it out work the drive up and down a few times, then snug it back. Worth the effort and cheap.
Thanks