Ford 641 three-point hitch leaking down

I have a 1958 641, and my hydraulic lift is "leaking down" slightly; and has done so for about 15 years. I don't notice it at all with lighter implements like my scraper blade, boom pole, etc...; but when I use heavier implements like my 7-ft sickle bar or three-point tedder, it will leak back to ground level unless the lift lever is more than 3/4 full position at around 1500 rpm. Also, when I lower these heavier implements, they don't go down slowly, but slam the ground.

I opened the top fill cap and looked down at the piston when lifting, and it doesn't look like it's leaking. The bottom of the cylinder is a little wet, and looks like oil is on it, but I don't see any flow when lifting or lowering.

What else might be causing this issue?
 
if there is no flow out of the cyl mouth when it is leaking, then your leak is in another section of the circuit. could be the
check valve, in addition to another leak.

Do you have any external hyds like top cover mounted remotes?
 
i don't have any other hydraulics other than the three-point hitch. Is the valve you're referring to the one under the plate below the seat, or the main pressure relief valve? How would I go about checking it?
 

I found this image in another thread here.

HYD_NAA_ckt_lifting_mkd.jpg
 

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