Added a 3rd function valve? Ummm, maybe something happened to the boom valve and it's bypassing. You're going to have to find a way to isolate the cylinders from the control valve to find out which is not working correctly. The pressure at the cylinder may not be the only thing to check. But, if you do it correctly, you can put cut-off test valves in your pressure gauge rigs and insert the rigs at your quick disconnects for the boom. Raise the boom, shut the tractor off, and close both test valves to dead-end the cylinders. Hopefully, you put the valves between the quick disconnect and the gauges. Test valves for isolation are a pretty cheap addition to a test rig if you're making one for pressure gauges. If the cylinders hold position and pressure, the problem is at the valve. You should see more pressure on the head-end of the cylinder than the rod end because gravity works. A small piece of thread tape can do a create of havoc in a valve. If you added a valve, it's possible a tiny piece of thread tape got into the system and lodged in the valve. If your fluid was low, it would be foaming like crazy, putting air in the system, and normally, when a hydraulic pump gets air, the whole neighborhood knows. It's LOUD. That makes things spongy, but won't really make them drift down when your valve goes back to "neutral" position. Air will compress, oil will not. The neutral position of the valve on the boom should block any flow to either end of the boom cylinders.
