You probably never adjusted the port reliefs on either side when installing the thumb. There is a simple single stage on the bottom of the valve for the B function. The valve that's on top is the 2 stage for A function. Consider this scenario. You close your thumb. Your spool is now centered, oil can't go anywhere. You curl your bucket and impact the thumb. You have 4300 psi or so to curl with. The oil in the thumb has nowhere to go. If the relief is set above 2500 psi, one of 3 things happens. 1, crack the piston/blow the packing. 2, bend the rod. 3, split the can. As strange as it sounds, the hose rarely fails. The relief failing is strange and uncommon. Some serious argument must have occurred between the two cylinders.