How to replace the mamuel relief value on a 90 hp yamaha out board motor

Some pressure switches have a valve that reliefs the compressor head pressure when the motor stops and closes the head pressure relief valve when the motor runs.
There is a check valve that keeps air pressure from coming back to the compressor head and the relief valve.
I think the check valve is leaking tank pressure to the relief valve and compressor head.
The check valve is where the pressure switch mounts on to and it mounts threaded on to the compressor tank.
Try replacing the check valve

the power trim is a simple hydraulic ram/s that hold the angle by having hydraulic oil pressure locked off by the control valve
if it keeps falling down that indicates either a problem control valve ( seat damaged, weak spring, adjustment)or a failed ram/ piston seal or a scored cylinder that is allowing the oil to leak past the piston/s
the relief valve would only come into operation when the unit reaches full extension of the ram/s to prevent overloading of the system
As it operates in adverse conditions, it is possible that there is corrosion in the cylinders allowing the bypass of the hydraulic pressure to happen
discuss the problem with a service center as it will not be an uncommon problem and they may have the answer as to what to repair/replace to fix it

Low fluid level will cause it to fail to actuate and not go up or down, can also let it not hold position as well. Filling to proper leevel with specified lubricant is recommended. If you allow the level to go too low, the pump will pull air and could burn out.

If it leaks constantly until the tank is empty or very low, the problem is most likely a leaking check valve. The pressure relief valves mission is to empty the pressure line between the chech valve and the sylingers to facilitate unloaded restart of the motor. The check valve is usually mounted directly on the tank an connected to the large copper tube from the sylinders as well as the relief valve through a smaller and thinner tube (often nylon). Empty the tank, unplug the power and open the unscrew the cap of the check valve and retreive the spring and the valve disk and inspect for damages, foreign objects or replace if needed. Refit it and test run the compressor. If your problem is that the relief valve leaks only while compressor is running and just after it stops, the problem is likely to be the relief valve itself. This is usually an integrated and rather delicate part of the pressostat (pressure, on off switch box) and you may need to replace it as a unit unless you can clean it by pressurized air or gently dismantle it for inspection and cleaning or removing foreign objects or dirt.

This switch is only rated for up to 2 HP at 240 volts and 1-1/2 HP at 120 volts. Using 3 phase power, you can go only up to 3 HP. That would not cause the problem that you discuss, but it is an unsafe condition.
Some Square D switches have a small relief valve so that when the compressor reaches pressure, the motor stops and the relief opens and lets pressure out of the compressor cylinder. Does your switch have that and might that be what is "leaking"? I believe that if the compressor is tight that the leak would stop rather quickly.

Do you have a pressure relief valve. Take off the cover, make sure all fittings are tight and their is no cylinder head leak

The problem is the seals or relief valve inside the pump or the cylinder seals. The fluid is suppose to stay where you pumped it. It is a closed system unless the outside and final seals leak on the cylinder. Then you would lose fluid.

You need to test the cylinders separately for bleed down. You would need a different source of fluid pressure. The pump can be hooked up to a fluid pressure gauge for testing.

If you do not want to deal with Marinas you may be able to get a Tractor place to test them. They deal with cylinders and pumps all the time. You can also save on replacement parts on the internet. Some of your parts may not be rebuildable. You can ask about the Labor beforehand.

Wish I could tell you a cost, but we really do not know what is broke or repairable.

To check the fluid level, tilt the motor up and engage the tilt lock lever. Open the bolt on top of the tilt/trim reservoir. The fluid should be even with the bottom of the bolt hole. If not add some fluid. When you put the bolt back in, run the trim up and down a few times. That should purge any air out of the system. Always be on the lookout for any leaking hoses or fittings. Hope this helped and best wishes.

Is the air "leaking" from the pressure relief valve? If so it is either faulty or it is doing what is supposed to do. The pressure relief valve is there in case the pressure switch fails.The tank could blow up if the pressure switch fails, so the pressure relief valve is there as a secondary measure to insure the tank does not receive too much air pressure. Assuming your gauge is operating properly, watch the gauge to see if it reaches the maximum pressure (check your manual to find the maximum PSI) or if it goes over the maximum pressure. If it goes over the maximum pressure then replace the pressure switch. If it the motor stops at the maximum pressure, replace the pressure relief valve.