Just replaced my valve that I referenced above. So far, 12 hours and no drooping/sagging of the front end. Fingers crossed that continues. It's a good sign though as my front end would be sitting on the tires after sitting overnight.
It's worth a check for sure. My front end was dropping at the same rate so I replaced the only thing that has both front spring air lines on it. Good luck with yours.
Edit: I did not hear any audible air leaks from the valve. You could try soapy water but listening alone did not yield any useful info. I disassembled it and it looks like the main seal had flattened and is not sealing but I can't be sure unless I pressurize it somehow.
