1994 mercury outboard trim leak down

[luckytempo93]

i have a 1994 nitro 170 bass boat and the trim unit will not hold the engine in any up position. it sinks till it hits the ground (when on the trailer) or in the water. last year i replace the fluid and cleaned the pressure relief valve between the motor and cylinder. i tested the opperation of the piston on the ram by jacking the lower unit up and and taking the weight off the weels on the trailer and it stays that way. so i ruled out the piston seals. what else is in the system that would cause that except for a weak pressure valve? http://www.crowleymarine.com/mercury_parts/360/300.cfm here is a picture of the trim unit i have - exploded