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Mercruiser used quite a few different trim pumps back then, and I do not know of any of them having a manual relief valve on them. Can you give me your engine serial number so I can look up and see what pump you have (hopefully it has never been changed) or is there any way you can take a picture of the pump.
More than likely the problem is in the valve body in the pump, but I want to make sure.
the engine serial # XXXXX(NNN) NNN-NNNN/p>
Lower unit Serial #(NNN) NNN-NNNN/p>
Still no manual relief valve that I see. There are fill and vent screws.
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Did you remove on of those?
That is not something that is ever adjusted or touched outside of the factory. That is why they plug them up.
When it won't go down, it is 1 of 4 reasons.
1. Resistance in the cylinders.
When you get this, it is usually the end caps on the cylinders that get corroded, and the trim rams "stick" in the end caps. You replaced the cylinders, but that is not to say the same thing can not be happening with the replacements as they were used.
2. To low or no pump pressure.
If the drive starts to go down after you kick it down, this is not likely the case. But in either event, this is something that would need to be measured with hydraulic gauges, a specialty tool from Mercury.
3. Low oil level.
This will do it to. Since you replaced the trim cylinders, you lost a lot of oil in the system. Sometimes it will take many, many cycles to get the trim system completely full of oil The anaology I like to make is your filling up a 2 gallon jug 1 shot glass at a time.
4. Control valve assembly inoperative.
Also known as the valve body This part is not user repairable. There are no instructions on the service manual for dissasembly or repair. Mercruiser specifically wants you to replace the valve body or the entire pump assembly
Thats it, right out of the book. Hopefully it ends up being those end caps or low oil, as anything else is going to be pricey to repair.
Post back here with questions.