1987-1993 Mercury Mariner Outboards 70/75/80/90/100/115HP 3 and 4-cylinder Factory Service Manual, Page 317Get this manual
Leak Down CheckPilot Valve Assembly 1Debris or chips between valve and seat (a), usually imbedded in rubber valve seat2Pilot valve installed from non-chamfered end of spool, results in nicked or damaged O-ring (b)3Nicked or deteriorated O-ring (c)A leak path is created between the UP side of the cylinder and the reservoirThe trim system will leak down until the trim port in cylinder is covered
Leaks Past Ball and SeatPiston Assembly
Unit will trim to full or near full down position and then will begin to trim up while trim switch is held in DOWN" modeIf trim switch is released, outboard can be pushed (by hand) down to the point where trim UP started1Inspect balls and seats in piston assembly for debris or damageRepair or replace ballsseats