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

[(F5)]

Yeah that is scary shot when it comes in.

Perhaps binding caliper is causing drag that overheats fluid?

Take pistons out and clean check dust seals pulling them back in, replacement if deformed. Sliding pins can be replaced as well as pad pins if grooved. Full pads will give more insulation.

Finally if disc is coned it might drag at top of pad for example . Check with straight edge.

[BMG]

Forgot to mention that lever pressure can fade while the bike is sitting in the garage or even just moving it around the garage. Rules out overheating, warped disc etc

[SS109]

Any weepage around any of the fittings? Are the fittings and bleeder fully tight? If none of those are found then I would suspect the seals in the master cylinder are bad.

[gasgasman]

Could be a bent rotor, pushing the pads in.

[(F5)]

An 18 seems so new. But if it has a bunch of hours maybe that's 5 years of wear.

I've come across master cylinders that don't improve with rebuild. The issue is an alloy bore and a sinusoidal lever push. The bore wears like an hourglass.

I bought a new mc for mine and all of a sudden after every other thing it came right.

I'd tried Chinese ones but weren't right. A new one out of Italy of all places was surprisingly cheap on evilBay.