" What is the "ideal" or best case hold time for an implement?"
I have 4 8N's & a 740; a disc will stay up for 2 days on one of the N's. It will drop to the ground in 30 minutes on another.
Before you start, see tip # 16 to make sure it's a top cover issue. It could be a leaky pressure relief valve.
And, before you start, it would be a wise move to get a jig from Zane Sherman to adjust the internal linkage. And, make a couple of “guide studs” by cutting the heads off of two 7/16-14 x 3 5/8” bolts. Also, if your lift cylinder rebuild kit has the leather washer, get CNH part number 87051231 which is the neoprene washer for the lift piston instead of the leather one.
The lift cover weighs nearly 100 lbs with the upper lift arms attached, & is very hard to maneuver. An engine hoist is an excellent idea. Remove the seat and spring, disconnect the upper lift arms at the knuckles. Remove the 14 bolts that hold the cover to the housing. (note that there are three different lengths) DO NOT remove the four nuts under the seat! Remove the right side inspection cover so you can remove the control rod tip from the pump intake valve. Lift the back of the cover straight up and slide a piece of wood under the cover to keep the tip of the control rod out of the rocker shaft. Once you are sure the control rod is out of the rocker shaft, you can then remove the cover. Turn the cover upside down, remove the four bolts that hold the cylinder to the cover, & use a little compressed air on the cyclinder to remove the piston. Check the cylinder for excessive scratches, hone if necessary. Install a new piston with "0" ring and backing washer in the cylinder. Install the CNH neoprene washer w/ the concave side toward the “O” ring. At this point, check the cam follower pin & the control rod. If the cam follower pin is worn replace it. Then, if you have a jig, adjust the linkage to specs. The ‘shade tree’ adjustment was to bend the control arm to compensate for normal wear in the linkage, springs & cam follower pin. If the control rod is bent, remove it & straighten it. (very carefully; they break easily!) Replace the gaskets under the cylinder and bolt it back on the cover. Put the gasket on the tractor (no sealer) and put the gude studs in place. Unless you do this every day or have eyeballs in your finger tips, get some help to guide the tip of the control arm back into the rocker shaft. (this is the point you will wish you had a engine hoist to hold the top cover while the control rod dances around the intake valve arm on the pump!) Align the cover holes up with the housing holes. Remove the guide studs. Tighten cover bolts, connect lift arms install the seat. Re-install the inspection covers with new gaskets.