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Built up a new road bike with flat mount disc brakes. Bled them and the lever was fine, few days later a bit soft, a few days after that the lever hit the bars. I've rebled a few times and they're fine for a few days or a week and eventually they go again.
There's no noticeable leak so I guess it's a tiny leak somewhere but given it's on both brakes I'm wondering where it might be. Is it worth replacing the olives and starting from scratch? Is there any way I can identify what is causing air to get in?
Did you disconnect both hoses for routing or shortening during the build? If so, did you use fresh olives and barbs for each? If not, I'd start there. The front and rear brakes are independent systems so whatever the cause, it's happened to them both.
Your other option is to bleed, clean and dry the system, then tie the brake lever back to pressurise it and dust with talc. That should help identify any really small leaks.
Fresh barbs and olives, it was an unbuilt set so I had to do everything and it's entirely possible I messed it up
Simply bleeding is not enough. You have to successfully bleed them 🙂
Well the LBS has also had a couple of cracks at bleeding them to no avail.
Do they fail after riding or after it's been stored? Do you hang the bike the bike by its front wheel when storing?