I got a bravo that slowly comes down over a few days if left trimmed up in the barn. When under power it will also slowly trim down and I have to keep adjusting it.
Is it the seals in the trim rams or something in the trim pump? What's the right way to know for certain?
Thank you.

If you don't see oil leaking from the cylinders or an oil slick on the water, likely not the rams. The pumps have a base plate that fluid is controlled by a series of steel balls in an aluminum seat. Those tend to leak after a while causing them to lose pressure. I thought someone made an oversized ball kit, but that's been years since I looked.

Its most likely the valves in the trim pump. I had the same issue on a boat years ago. Replaced pump and all good.

Is pump replacement the only option for repair? Or is the valve base plate available and replaceable? Or should I just replace the pump regardless since it is 28 years old...

I would just put in a new pump. I also used SEI Marine rams on mine, much cheaper than Merc, and 4 years later they still seem just fine to me.

https://www.sterndrive.cc/product/Fi...-98704A26.html

It's by far the easiest! I've cracked open several pump bases without roaring success. Some work for a while, others never worked better, sometimes you drop a valve ball and throw the whole thing in the trash (long story) If you look around CL and Ebay, there are some that come up relatively inexpensive.

28 years it doesn't owe you anything

Thanks for the help.