Replaced bad master with a less bad used one.
New master piston kit.
Cleaned and reassembled all calipers. New seals on one that had a torn dust seal.
Bled the *** out of it. forward, backward, upside-down and sideways.
Pistons seem to be retracting too far. The calipers float, but then float back. Lever is slightly firm, but pulls all the way to the grip. Not nearly good enough.
Clamping on the old lines just above both calipers made the lever still spongy but possibly tolerable. Releasing one made it a little scary feeling. Releasing both, and definitely not okay.
New stainless braided hoses from a quality manufacturer. Same thing. Must have bled old and new right the first time around. Seriously, we've bled them with almost a gallon worth of fluid by now. It's NOT trapped air so please don't suggest that.
Removed calipers and bled upward with the pistons blocked with wood. When pumping pistons out toward the blocks, they retract visibly each time.
Same feel whether clamping on blocks or rotor.
What it isn't:
Trapped air. I know how to bleed brakes, and that's not the problem.
Bad master. Clamping the old hoses showed the master to make awesome pressure almost immediately . Rock hard before 1/3 pull.
This is not bad wheelbearings or excessive rotor runout causing the pistons to be pushed back in.
Binding caliper pistons are relatively common after a rebuild, particularly if you used aftermarket seals like K&L brand. Try tying back the lever to the bars after pumping up the brakes and squeezing as hard as you can. The constant pressure in the system will allow the caliper pistons to creep out towards the disc and reduce the amount of fluid needed to get the pads in contact with the disc. If that fails, and you used aftermarket seals, I'd order OEM seals and rebuild again.
Awesome illustrations, and thanks for some well informed ideas!
We used K&L part kits. Always heard they were very good, second to OEM. The brakes don't really "pump up". The lever stays squishy right up to the grip with gradually increased resistance.
I'll try pumping and tying, and hopefully the pistons will creep in the seals. I'm not particularly hopeful though because we've tried testing and bleeding the brakes on different thicknesses of blocks, so the piston was already forced to move past the seals in both directions.
If they keep sticking (assuming it isn't some other problem), maybe I'll try overextending them just a touch so that they retract to the right position or just shy of it, then muscle them on. The pistons aren't binding, just would need to be pushed back in their bores a bit, so that might work?