I have a '91 7.4 330 merc bravo I w/ 400 hours in a 230 Powerquest.
Recently, the Drive is drifting down to the ground. I trailier up coming out of the water Drive home and it sags down a 1/2"-1" in an hour. within 24 hours it will drift all the way down to the ground.

With my occupation, I repair and trouble shoot alot of big complicated Hydraulic systems Ranging from 500 hp pumps, servo valves, closed loop controls, cylinders, Motors etc..

While I have no experience with the trim cylinders, My question is what is the most likey problem component.
Clylinder OR Valve

At this Point there are no visible external leaks as I just cleaned the bilge and it is spotless. And no signs or puddles in the drive.

I think it could be one of the cylinders leaking across the piston. OR it could be the Valve letting the oil go back to the resevoir through the non-rod end hose.

So based on the experience of those who work on them often, What your bet?

If it is a Clylinder, Can I get the piston seals to rebuild it?

I had exact same prob in my 1993 35. Replaced trim pump. Or you can look here and try rebuild kits.

http://www.go-fast.com/

Often, the inside bore of the trim rams becomes pitted and the seals hardened. Add em together and the drive sinks down.

One vote Pump/valve assembly
One vote Cylinder

I'll be looking more closely to it this weekend.

Thank you for the replies

here is my old thread with good info.

http://www.offshoreonly.com/forums/s...highlight=trim

if you do a search for "trim" titles only there is dozen to view also.

If you can locate inverted flare caps and/or plugs easily you could extend cylinders and plug. Check the next morning. Before you start throwing parts at it.
Jim

Well I think I figured it out. I cleaned the pump / valve assembly, draind all the fluid from the cylinders and lines that I could and it seams to work fine now. I am thinking it was just dirty or sticky. Didn't really find the smoking gun.

Tested the cylinders - I raised the unit up, placed a floor jack under it, and removed the lines from the cylinders that raise. At that point I plugged both cylinders and released the jack. It held up for 24 hrs with no sag. But the cylinders were Isolated at that point and I didn't know for sure that both were OK. So I removed one of the plugs for 24 hours - no sag, Then the other- No problems with the cylinders.

I then took the pump out of the boat and completely disassembled it. It was pretty dirty with alot of grime on the bottom of the resevoir. I cleaned everything with heating oil kerosene Then blew dry with compressed air.

I pulled the cartridge valves on the side. There was no apparent damage and barely had a ring where the they seat. In between the cartridge valve is a pluger that unseats the cartridge check valves. Although I didn't remove the plunger, it moved freely.

On the bottom of the pump there are three relief valves. One for each direction and a third that appears to be the drive unit blow back. The two are adjustable, I measrued their hieght with a caliper before taking them apart to be sure they end up where I started. The third just bottoms out in the main valve body.

Two things to note here: the adjustable reliefs had different size balls in them and although the springs appeared similar, the preload hieght was drastically different. So swapping the lines and wires around when problems occur is not the right thing to do. Secondly the main blow back relief valve is only on the lower circuit. So damage could be possible if swapping lines and wires.

As with all hydraulics be sure to get all the mating pieces back together in the same order. They tend to wear into each other and problems could arise if not assembled the same way.

I refilled the system with west marine trim oil. - hope merc is ok with it? Then ran the pump up and down several times to bleed out any air.