I've worked on too many excavators. I hate getting this swarm of "I'm so wrong" answers, but let's see what finally fixes it. See, if his valve leaks a little, and a leaking piston puts more pressure on it than it can handle, it will leak down just as he says. Whereas if the piston didn't leak, it may slowly leak down, but would be at an acceptable rate. I can get the cylinder packed for 40 to 60 dollars. Valve is a lot more if the valve leaks it will allow the displacement you say it requires to leak down. Being that the boom cylinder is the most used one on it, I stick with my previous answer. I'll spend less than 100 on that cylinder before I do 1000 on the valve. Maybe you would spend 1000 on the valve and then less than 100 on the cylinder? I've made my whole living with a service truck spending other people's money. Just a certain way I've learned to go about things. Road graders, dozers, skidders, Trac hoes, back hoes, I do not think it has ever been the valve. Sure, I've had some re worked, because they were so absolutely sure that the valve was the problem. It did the same thing, I re worked the cylinder and it was ready to go.