I hope I am not digging myself in deeper here ( and/or developing a reputation for mental density that I probably richly deserve... )
It has been pointed out to me that the model I described earlier does not cover the situation where the tractor is used to pull or push on the backhoe ( This applies equally to any hydraulic implement, FEL, etc. ) In that case, force is applied to the cylinder and thus to the fluid within it, rather that the fluid applying force to the cylinder and from there to the outside world. Although there is always resistive pressure be applied against the cylinder in any case where work is being done, there are two distinct scenarios to consider.
In one, the cylinder is the simply being used to force an implement to act on something that is otherwise static. An example of this would be when pulling the bucket of a backhoe through the soil. In this case, the only force in the system is that supplied by the hydraulic pump, via the hydraulic system, to the cylinder. There can be no more force than the pump can provide and as soon as the valve is closed, there is no further pressure. In this case, the relief can be anywhere in the system, as long as all of the components are rated accordingly.
In the second scenario, the load on the implement is actively pulling or pushing against the cylinder. Two examples of this are a load lifted by a backhoe or FEL, and hooking a backhoe on something and then pulling with the tractor. In this case the dynamics are seemingly the same as long as the valve is open; a relief anywhere in the system will limit the hydraulic pressure. However, once the valve is closed, the hydraulic fluid on the implement side of the valve remains under pressure based on the force that continues to applied to the cylinder. If we are talking about holding something up that was lifted by the hydraulics in the first place, there is no problem. Only the gradual leakdown around the seals needs be considered. But if the an external force, such as the tractor moving, or additional load being added, is increased, the pressure will be increased as well. If a relief is not installed downstream of the valve, this can easily exceed the limits of the components of the implement. If a relief is provided, how the implement will move once the allowed pressure within the implement is exceeded must also be considered.
Peter
"No tractor of <font color=red>a</font color=red><font color=green>n</font color=green><font color=blue>y</font color=blue> <font color=orange>c</font color=orange><font color=black>o</font color=black>l<font color=yellow>o</font color=yellow><font color=purple>r</font color=purple> /w3tcompact/icons/sad.gif, just <font color=green>green</font color=green> with envy"
