Re: 120 gearcase slowly lowers, not the tilt cylinder - think its valve, now what?
That is almost NEVER the cause of leak-down. Under the pump are two check valves, hopefully the screw in type. the front check valve is the UP check and usually the very small O ring inside it fails, resulting in the engine slowly (and progressively faster) dropping.
The tilt cylinder has a large O ring on the internal piston. Wear or failure of this O ring will cause the engine to drop, again, slowly at first and accelerating as wear progresses.
NEITHER condition will show leaking fluid outside the tilt/trim system.
The small O ring can be had as a Tecumpsa carb repair kit. search the forum to find a part number. The large O ring on the tilt cylinder piston is PROBABLY a standard O ring available at Graingers, McMaster Carr, and other hardware stores.
Since there is no evident fluid leakage I focused on the valve. My valve body photo
