Help troubleshooting Tanke hydraulic disc brakes

[Agrajag]

Hi all,

I bought a set of Tanke TB-800 4-piston brakes to upgrade an old bike on the 11/11 sale and they arrived this week. I've installed the front one (haven't got the back one on yet) and am having a problem that Google isn't helping me solve.

Basically, the pistons seem to be overextending and not backing out again. Within 1-2 pumps of the lever, the pistons extend so far that they permanently engage the disc and the wheel can barely move. If I take the wheel out and push the pistons back out, as soon as the wheel is back on and I pump the lever, the same thing happens again. With the wheel off and before pushing them back in, the pads rest probably about 2/3 of the thickness of the disc apart whereas brakes on my other bikes would be fractionally wider than the thickness of the disc.

They are easy to push all the way back out with the wheel off, so I don't think there is too much fluid in the system, but just in case I tried loosening the lever bleed screw and pushing out the pistons to the wide-open position with that opened, which ejected a small amount of fluid. This made little or no difference to the piston extension issue. Figuring that I may have now gotten air into the system I did a quick lever bleed, which got out a few bubbles of air but again made no other difference. I haven't bothered to fully bleed the system as yet as to my knowledge that is unlikely to do anything useful, as dirty fluid or air in the system shouldn't be causing this problem.

Interestingly, this brake arrived with the pistons so far out that the little spacer they had in transport needed a pair of pliers to pull out, whereas the back brake lets me easily pull out the spacer. I'll see if the other side has the same issue when I install it tomorrow, but in the interim was wondering if anyone here has any ideas about what would be causing this issue.

Have I just got a defective brake or is there some easy fix that I'm not thinking of?

Thanks in advance!

[Agrajag]

An update - still haven't been able to fix it. The rear brake is installed and works like it should. I've also done a full (top down/gravity) bleed of the problematic front brake, which has made no difference to the overextension issue.

I don't fully understand how self-adjusting hydraulic brakes work, but the only thing I can think of is that there is a return valve somewhere in the system that isn't working like it should. Am guessing the problem is on the caliper end, not the lever, given it was locked down on the spacer when it arrived with the lever not yet attached.

Whilst I'm probably just going to replace the problem brake (and try to get a 50% refund from the seller) I'd still be curious as to whether anyone else here can figure out how this is possible (and I'll keep the spare for a while in case someone has an idea about how to fix it - fully disassembling and rebuilding it is probably beyond my skillset, though).