Nice job. On having another good look I spotted the O ring t the top of the piston tube. Looks a pig to replace. But I think I may have found the problem. Looking at the top plugs from the rear the LH one came off and below it there seems to be some sort of rod. Now the distance between the top of that rod and the base of the plug when tight is 4.5mm. I reckon that is the overload release mechanism. Now the right hand plug has indeed got a ball valve but there was no spring in there!. Now the distance between the top of the ball and the end of the extended nose of this plug was 2mm. So I think the spring has been lost in the past by persons unknown. Trouble is knowing what strength spring to fit know! I believe this release is for when you fully extend the piston and keep pumping it senses the pressure and dumps well before the overload mechanism. The seals all look good as the ram was not leaking or showing signs of leaking so first I will go down the spring channel to fix and then if it does not work or leak sort out seals. Any ideas on spring length, dia, strength?