I recently had similar issue...
on drive to work everything seemed fine. Stopped at local coffee spot. When I pulled up to light leaving coffe shop... pedal went straight to floor. Eased to office mile or so away. No discernible leaks, but fluid was noticeably down. Add some fluid, pump pedal a few times things seem fine. Two days later front brakes are draggin like they won’t release.
Take to local shop to use their lift and inspect... they have lift down so I leave car for them to check out. Long story short they mis diagnose clutch master and slave cylinder leaks as cause only to need master cylinder in end. Not happy but now I am ordering steel braided lines to complete my unplanned brake system upgrade.
new brake master cylinder, Clutch master cylinder, clutch slave cylinder, clutch pipe, brake disc rotors and pads.
Rmeuropean.com had everything I needed