[Lowrance]

Just as an experiment I loosened the caliper mounts to avoid any pressure that would tend to push the pads apart. Zero movement of the calipers when the brakes are applied, which is disappointing since it's one of the few other plausible explanations, heh. Lever free play is adequate, there's zero pressure on the piston until there's a bit of lever travel, and when there's sufficient pressure, the full travel of the lever ends well short of the bar.

It's just that after no lever squeeze for a minute or two, the first pull is very soft. The second pull is firm and behaves exactly as it should. It's almost as if the pads got slightly pushed apart in between, but I'm confident they're not.

The shop seemed pretty convinced it's a master cylinder problem, but.. this is a brand new one, new brake fluid, bled at the calipers and the master, etc.

Unfortunately the last piece of the puzzle is the ABS pump. If it's piston/seal is leaking you'd never know it externally as the fluid would just bypass it.

Hopefully a good bleed with the dealer tool sorts it for you but go in prepared for the worst.

Good luck!

 

[Lowrance]

Sure, and that's the working hypothesis, that the piston doesn't seal against the master cylinder wall, and pushes past the fluid. The only objection is that it seems weird to get two bad ones in a row. Still going to have the shop go over it again with the new m/c, it's worth spending another $100ish for some diagnostic work from people who actually know what they're doing. (hopefully)

Not the master cylinder. The ABS pump has pistons as well. It's completely sealed, no resivoir, so piston will just move thru the fluid in lieu of pushing fluid if seal I'd damaged or held off cylinder wall by debris.

 

[Lowrance]

A damaged seal will allow fluid to bypass the piston so instead of forcing fluid down to the calipers, the fluid doesn't get displaced. The piston just moves thru the fluid in lieu of moving the fluid. Hmmm, guess it's hard to describe.

The fluid doesn't actually have to escape if the piston can't push it due to damaged seal.

 

[Lowrance]

The front brake line goes from the MC to the ABS pump, another line runs from the pump to the splitter under the lower triple clamp and then down to the two calipers. If there is any kind of foreign material or debris in the brake fluid it can foul the ABS pump seal.

What year is your bike? Has the brake fluid been flushed at interval spec'd in manual? If not, debris can foul up the pump. That was the issue I had with my 2014 S1000. As I mentioned, I bypassed the ABS pump as I was not willing to spend $2k for a new one. Bike was less than a month out of a 3 year warranty which just added salt to the wound.

Anyway, hope you get it sorted.

 

[Lowrance]

I had a similar issue with my BMW S1000's rear brake. Replaced MC but didn't sort the issue. Took it to dealer to get bled using BMW computer where it was diagnosed as a bad ABS pump. At ~$2k, told dealer no way. Bought new rear brake line and bypassed the abs pump which worked fine.

You can try KTM shop for bleed but will likely have same issue. Maybe it's the ABS pump and you want to retain ABS so you buy new pump. But you also have an option to delete ABS with new brake lines directly from MC to calipers for a fraction of the price.

Good luck.