Had the same deal earlier in my "What's a diesel" education. I'd done some unrelated work to the fuel system but had disturbed the lines which were old and had weakened at the clamps and started leaking.
Is the line to the drain/T valve wet at the valve and leaking which is running back down the line to drip into the valley? Could be the Valve has failed but that's a real long shot. More likely it is the line has weakened at the valve or at Fuel Filter Manager (FFM). Same with the other rubber fuel lines at the FFM and IP.
While you're at the front of the motor, Check the short rubber injector return line at the front of the motor to IP for the same issues.
Power up the lift pump by jumping it at the fuse/relay box's A or B AUX terminals or key on till the LP stops ticking. Then check the lines for drips while the system is pressurized.
Repair by removing the clamp and the line and inspecting for wear and tear and if the line is leaking at the clamp, trimming back a quarter inch of old damaged line at the connections and replacing the clamp with hose clamps and not the spring loaded factory ones that are prone to fail.
Same with the other lines at the FFM. If it is that short injector return line to the IP, you'll be better served replacing the line altogether as it is only a few short inches and not a lot of room for trimming and refitting.
For the FFM, Remove the 6.5 cover or "Dog House" as it is often lovingly referred to, that sits over the intake manifold if it is still there. 4 10mm bolts IIRC.
Remove the two bolts holding the Fuel Filter Manager on the lower intake manifold, 15mm IIRC. There should be enough play in the fuel lines to lift it up and inspect the three lines from the fuel tank/LP, the line leading out to the IP and the line leading out to the fuel drain valve at the alt. Pressurize the system and check for leaks, trim lines and replace clamps as needed.
If you want/need to replace the lines under the lower intake it’s as easy as using the old lines to pull new ones through to the IP and the T-Valve.
While you’re back there looking at the FFM lines, check the return line to the tank as there is a rubber hose that makes the bridge from the hard line in the valley to the hard line that starts over the bell housing. Clamps or hoses there could have failed as well but with the way you described the issue, it may only be a point worth inspecting.
Pictures might be of help too.
