I have a 2001 90hp Yamaha. The other day I was putting it back in storage and raised the motor to remove my transom saver. The motor raised up fine but started slowly falling back down. So, I raised the motor back up and the motor would slowly fall back down. Any Ideas ??

Big C get a manual for that motor and it will tell you how to check the hydraulics of the system in a troubleshooting tree type format but you more than likely have check valve problems or leaking o-rings in the trim assembly. It depends on at which point it's leaking down is it leaking down from full up position to full down position or does it stop and hold at the trim cylinders from a tilted position. The cheapest way i was told for my merc was to start with the complete o-ring kit and check valve for the tilt cylinder then take it from there on down to the trim rods check valves and then last would be the check valve in the pump which on a mercury you can't replace you have to buy the pump in one assembly you might think by going through all of this you probably would be better off buying a brand new one but it wasn't the case with mine a brand new one for mine was a little over $1200 with shipping but by doing it myself it was about $750.Good luck!!!!!!!! Not sure about yamaha systems but in one way or another they are probably set up kind of the same way as other o/b manufacturers.

I would open the manual release valve about two full turns and then run the trim motor up and down each way for about 30 seconds. This will clean any debris that may be stick in the release area causing the trim unit to leak down. Then tighten up the manual release and run the motor up and see if this helped. Might have to do this a couple of times to wash out the trash that might be the cause of your problem.

This is a problem I see many times unless you see a leak in the trim unit somewhere. I have seen a few trim units I was not able to fix this by loosening the manual release valve and R&R'd the manual release valve with a kit and that fixed those. Maybe the trash is all that is causing the problem and yours will be as simple as opening the release and cycling the trim pump and then tightening back down. Make sure to tighten the valve really snug about two or three times before leaving the valve in the tight position also... Let us know if this helps PLZ.

I have had several Yamaha's. On a couple of them the cap which the strut comes out thru has 4 indentations for a spanner wrench . I have had that part work loose and cause a leak.. I was seeing a oil slick in the water when I was fishing.
The final cure was to clean that area and put lock tight on the threads and had no more problems.

Sounds like an o-ring to me. I replaced one on my 2002 Yamaha. It was doing the same thing you mentioned.