Mercury tilt trim bleed issue

[Meano]

Having problems bleeding a mercury tilt trim unit.
Unit will go up but after removing reservoir cap, to check level and add fluid, it will not
go down without opening the relief valve. Afterwards it will operate up and down, but as soon as I open the reservoir cap it will not go down. I have done this at least a dozen times. Seems like the air should be out of the system by now. Anyone know why this is happening?

 

[Texasmark]

Don't have a TT schematic handy. Ram all the way out means you have the piston all the way up and the cylinder is full. Down position is trying to force fluid back in the sump and with the plug in it has resistance. Is it because the motor (common to up and down) or one of the relays powering the motor can't supply enough current to develop the power to overcome the resistance.

Thinking as I type I would go over to the TT relays and swap them. Then see if it doesn't want to go up....if so, you need a new down relay....grass is green, sky is blue....id wires are either green and blue or wires have a green or blue stripe. You want the relay with the green wiring. Otherwise there might be a bypass valve not opening but I'll go for the relay or you might have a connection corroded in the green wiring causing the problem....but swapping relays would tell you relay or corrosion....if one of those are the culprit.

Besides going up is where the pressure is the greatest and air in the system would manifest itself in mushy "Trim" under power.

 

[Chris1956]

Gee, what year and HP motor are we discussing? 2 ram, 3 ram or 1 ram PTT system?

 

[Meano]

It is a 1998, 175 hp. 3 ram system

 

[Meano]

I don’t think it is a relay or wiring. Once I put the reservoir cap back on and manually lower it, it will go up and down all day long. I’m thinking I could go ahead and operate it like this, but it just doesn’t seem correct.

 

[Chris1956]

Sometimes when you run them all the way up, they need a nudge to go down properly. Try not running them all the way up, and see if that cures your problem.