So. freshly bled brake, fresh pair of pads. Hope thing with 2 pairs of pistons fwiw.
And a floating rotor.

Sometimes it bites when the lever is pulled only a little.
Other times it doesn’t bite until the lever is almost touching the bars.
But then it can go back to good behaviour moments later… It’s just not predictable.

I can’t figure out why! What is wrong with this thing and how do I fix it?
Even if the pistons are moving unevenly the bite point shouldn’t change, right?

Is it bled right – would be the obvious question….

There’s still air in the system sadly

Try reverse bleeding (if you haven’t already), and then make sure you top the reservoirs up as much as you can before finishing (if the fluid level is low, when the pads wear it can cause air getting into the system).

Well we forced a lot of oil through, from bottom to top. It feels like once it bites for a given
Wheel position it is consistent until the wheel is rotated again. Maybe because the pistons get nudged a bit
As the wheel rotates? Is that consistent with the idea there is still air inside?

I had a problem similar to this before and it was down to a hair line crack in the reservoir top cap.

I guess the same could happen if the top cap or rubber diaphragm is not fitted correctly. Have a real close look at the top cap and make sure it’s tightened down sufficiently.

Other than that , take out and refit the diaphragm and refit then re-bleed the brake. Make sure the reservoir is topped up and that you’re ‘rolling’ the diaphragm into place as per Hope’s instructional video.

Thanks epic (presumably the same chap I have bought bleed kits from before) I will try that.

Another piece of diagnosis that a mate of mine thought this wasn’t air related, because when the brake did bite, it bit solidly rather than being squishy.

On occasions I have ridden with a slightly lose qr. This effectively knocked the pistons back if I was of the brakes, say for a rough climb. But if I was on and of them on smooth ground, coming down a twisty tarmac link, they would go back to normal.
So is everything else straight? Rotor, bearings, all properly tight?

Well we solved it. Changed a few things so not quite sure which one was the magic bullet…

1. Bled again. We noticed the way the caliper was mounted, the hose was connected to the bottom end of the caliper so presumably any bubbles in the caliper could not rise out. So we took the caliper off the bike and bled it such that the hose was above it at the time. Tapped it with a screwdriver plenty of times too.

2. made sure to roll the reservoir cap on properly (I had forgotten this last time) also tightened the cap bolts properly (hadn’t done this before as the bolts were worn out – so replaced with new ones).

3. I only saw tracker’s suggestion today but did happen to notice my rear axle was too tight. possibly the opposing locknuts each end weren’t opposed enough so the axle tightened itself as I was rolling(?) So the opposite problem to what tracker describes but if the axle was tightening/loosening itself then something like that might have been going on.

Either way, all fixed thanks for the help guys 🙂