1990 Yamaha 85 T&T problems

This unit bleeds down so fast that I have to run to the back of the boat to lift the stop when I want to lower it. It runs up and down fine it just won't hold pressure. No apparent leaks so it must be internal. I have checked the front check valve, the manual release valve and the fluid has been checked first then replaced. It has been sitting outside in Colorado weather for the last 10+ years and I just changed the trim piston rod wiper seals this year too. It not only bleeds down the tilt but when it hits the trim pistons they also bleed down slowly until it hits the stop. I thought since it seemed unlikely that all three ram seals were bad that it had to be something they all had in common. Not the case so far. I switched over to Dexron fluid on a suggestion I saw here, should I try some tranny stop leak too?