...dont forget the larger the rod the less force the cylinder will have pulling, ...
I agree with this but I don't think it will make enough difference to worry about in general use. Look at your loader cylinders for example. They are REALLY subject to direct forces. The top link is different as it has help from the lower arms, therefore don't have nearly the direct forces as the loader cylinders do. Using 3PT attachments, the lower arms do most of the "pulling" and the top link is primarily for positioning and stability. Plus if you put a dual operated checkvalve on the cylinder it's going nowhere. Tilting the implement such as a boxblade should be easily done even with a wimpy cylinder because the implement will "revolve" so to speak, around the lower lift pins. Holding in that position can be accomplished with the check valve. I'd want a large diameter rod to reduce rod flex or possible bending of the top link.
...which is the direction the force usualy is on a hydralic top link...
I disagree for the following reasons. The implements are secured to the lower lift arms via lift pins and also to the top link cylinder. When the implement is raised off the ground and manipulated (tilted) by the top link cylinder, that's the only time it pulls the implement, trying to rotate around the lower lift pins. Pushing the implement down (tilting backwards) is easy because it's weight helps. Pulling the implement (rotating it up) requires the cylinder to retract and as long as the cylinder has enough rating, more than the weight of the implement, it's going to do that. Each cylinder will have a chart of how much it can pull or push and I bet none of your implements will exceed the force exerted by the cylinder pulling or pushing. (Unless you get a super dinky one of course)
So under use, all "towable" implements like the brush hog, a trailer, etc. will subject the cylinder to this. But the lower arms do most of the pulling. Now, ground engaging implements like boxblades and rear blades are different. Once the implement engages the ground, the tractor is going forward and the boxblade, for example, is giving resistance from use of it's scarifiers or blade. The resulting force will be straight back or even a rotational vector force caused by when the implements wants to rotate upwards around the lower lift pins. Guess what force the top link sees when that happens? That's right...it wants to compress. That means it's working against the "non rod" side which has the greater surface area. And if you had a check valve, it shoud not budge regardless. An example would be to hook up your boxblade to the lower arms only, without the top link attached. When you hit a rock or stump, I bet it wants to flip up? (rotate around the lift pins)? I think plows will tend to be dragged equally by the lower lift pins and top link except the occasional bump which will try to raise it. Another example to show the force go against the lift arms is when bulldozing with the boxblade in reverse. That will try to stretch the top link cylinder open because the blade will try to "fold" under the lift arms. That is why you bend lift arms, or in my case I've bent my side links doing that.
If the implement has resistance from above, like when you attach a chain to the top of the box blade. the opposite is true, and now the force is pushing against the "rod side" or trying to stretch the cylinder open. This is also seen when pulling something directly by the top link, which I would not do. And also when using a 3pt backhoe. Digging the bucket in, trying to stretch it open, will exert pressure on the rod side. But it is better to have a subframe anyway.
Please keep in mind that these are my opinions derived from my experiences and it doesn't necessarily mean I'm correct. I'd like to hear what others have to say what forces the (hydraulic) top link sees?
Thanks,