Hey folks, I've got a few of the old style merc trim pumps in the boats I own and once again I have another trim pump that is letting the drive fall (boats are trailered) over a couple days time. It's getting to expensive to keep replacing one every year for the same issue so it's time to learn how to fix them.
What causes the pump to slowly let the drive down and how do I fix it? No leaks and when you put the drive up you can stand on it for an hour and it wont move. Walk past the boat couple days later and the drive is on the ground.

Are you sure that your cylinders are not internally leaking a bit? Wont drip on the ground but will slowley lower as the oil goes to the other side of the ram. Not sure of a way to test but I woul unhook the lines, rig up some sort of air quick coupler with a gauge. Pump it to say 100 psi, and it "should" stay there. Test both side of the cylinder. (one at a time tho, LOL)

I had one years ago doing the same thing and my buddy at Stinger Boats took it apart. In the base there are a couple of balls that act as valves in that plate. The issue arises when the steel ball wears on the aluminum plate. The hole in the plate gets worn bigger and the ball won't seal it very well, so it bleeds past and tah dah! it slithers down. He said there was no way to repair at that point, so I just bought a new one. Maybe someone has devised a way to repair them, and they will chime in. Kinda of expensive replacement for a little worn aluminum!

Just get bigger balls!!

I couldn't help myself

Be Safe

3pointstar

Raise the drive and remove the lines and cap the fittings on the cylinders and you will know if the rams are bleeding by.

It's a good place to start but the rams are fine. I replaced the pump that went bad with a back up pump I got off of eBay and it's fixed.
The reason I ask is like I said, this is not the first time this same symptom happened. The pumps run and work fine but they just let the drive fall down over a couple days. Got two of them on a shelf now with the same issue. Figured if I would have learned 10 years ago how to fix them it would have saved a fortune form all the ones that went to the dump over the years with the same issue.

Yeah , just tossed that in because of the previous ram mention. Sounds like one of my favorite pass times at work , failure analysis, if your going to toss it anyway rip it apart and see what gave out and if you can do anything to fix it should it happen again, or keep it from happening in the first place.

That's why I'm asking. I figured someone has already went down that road and knew what was going on. As many of these pumps that are out there it surprises me on how little on line there is about them. For what they cost I would rather not treat them like bic lighters but if just finding info is this scares how hard would it be to find parts.

The valves in the pump assembly start leaking.

Merc sells valve kits, but I'm not sure what is included in them
http://www.mercruiserparts.com/Show_...HYDRAULIC+PUMP

Hey now there's a start. It's not the one you posted Griff but the older style but it's on the site. http://www.mercruiserparts.com/Show_...LY%2C+COMPLETE
Man that plate is expensive. Next rainy or cold day I'm going to have to do some exploring inside a pump. Maybe I will just see what is wrong.