It does sound like it could be the HTOB leaking.
Wedging the clutch pedal at full deflection with a 2x4 against the seat overnight should reveal a pretty good leak point if it or the lines are leaking.
If you don't have one, a Bluetooth Endoscope is invaluable for troubleshooting, retrieving part numbers, etc...
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1