250 pro xs trim/tilt question

[Procraftrob]

2010 Mercury 250 pro xs 1B839428
My trim/tilt is leaking down. I've ordered the complete O-ring kit and proper tool to replace. My first question is how do I know if it's a failing O-ring or the check valve. My second question is if check valves are failing how do I remedy that problem?

Thanks

[EuropeanAM]

Mercury offers a moderately complicated pressure gauge assembly with adapters, so that pressures can be monitored. That being said, most commonly the problem is in the Manual Relief Valve, Seal Leaks or the Port Side Trim Ram relief valve.

The other relief valves are installed from the sides of the unit (and sold separately). O-rings/seals for them are included in the Repair Kit.