I asked about the 3rd function because they can easily blow the seals on the backhoe if installed wrong. You eliminated this since you said you don't. Well I would rest the backhoe bucket/outriggers on the ground, remove the cap under that first(boom) valve stack (Allen key) 10ftlbs. There should be a lip seal under the spring position cap. I would look for anything out of the ordinary. From my experience they usually leak from overpressure of the valve.(dragging/driving with boom down/miss installation of other hydraulic valves) Most of the ones I have seen bend the top plate that holds the seal down. The seal for the bottom should be similar. I would make sure I have the hookups correct on the machine end. I assume you have two hoses. One P(IN)from loader and one PB(out) to manifold/threepoint The valves have one or two reliefs each. The reliefs look like big adjustable plugs(one at top and bottom) between the hose outlet side and the spool itself. These are usually set in conjunction with the relief on the source manifold on the Backhoe valve stack.(I Wouldn't play with them unless you know-have the correct tools). The silver cap at the bottom is the spool spring positioning cap. They usually contain some return springs/ball/detents. There should not be external leaks, even if the valve relief is dumping. I do caution you not to loose any of the parts, as many are not available separate from a whole valve.$$$$ The seals are generally available as I saw from the part diagram.
It is odd you and another member both have the same issue. I wonder if these are being installed right at the dealer. Your backhoes are plumbed to the front manifold/loader valve right? Not some funny rear remote business. These add on backhoes need a good shock free source for PB hydraulic flow. Any shock loads can blow the seals. This is why they are usually plumbed second to last in the hydraulic circuit before the three point.
