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You have the trim with the valve body on the bottom of the resevoir/ motor. There isn't a lot you can do but here are a few things.
Take off the hoses from the pump. Put a large pan under to catch the fluid. Disconnect the electrical.
Put the trim assy on the work bench. Drain all the trim fluid through the fill hole. Turn the unit on it's side and remove the 6 screws holding the valve body on the bottom. be careful of the large O ring.
Remove the valve body. Using spray carb cleaner spray, clean the valve body. There is a silver 1/2" "canister", pull if off. Clean the screen underneath. Use a small screwdriver and turn the gear where the shaft from the motor index. Use the carb cleaner while you turn. What you want to do is to remove any debris, dirt and old fluid. Don't disassemble the unit, they are not made to come apart there is no service procedure.
Once it is clean reassemble and bleed the system using the bleed screrws on the trim rams. You can use 20 W oil or power trim fluid or Dexron ATF. But don't mix them.
Do this and post back your results.