2018 xc300 - front brake disappearing and needs constant pumping

[BMG]

Was recently riding down a long fast down hill and the front brake completely went. No lever resistance what so ever. Looked for obvious leaks etc but couldn't see anything. Left the bike for 10 mins and pressure came back. Rebuilt master cylinder and bleed brake which didn't really make a difference- still had the issue of good pressure one min, then 2 mins later, nothing at all. Constantly needing to pump the lever to get pressure. Rebleed and was a bit better but still same behaviour. Changed brake hose in case but I still experience loss of pressure especially after a few mins of not using the brake. I'm out of ideas! Any one else experienced this and/or know of anything I can try to resolve? Brake pads are about 50% worn so that shouldn't be it.

Little bit scary not knowing if the brake will work or not!

Cheers