Can Grit back into Master Cylinder Causing Never Ending Big Bubbles?

[lostkiwi]

I had my left front brake overheat so to check it was indeed a caliper seizure I removed the caliper then pumped the brake a little. Only one piston moved and the other was stuck.

I blocked the moving piston to check how stuck the other was and it did move but wouldnt retract at all which I guess was the overheating problem.

 

I then pushed both pistons back in, disconnected caliper and went down the road to exchange it for a reconditioned one.

 

After installing the recon caliper and commencing bleeding it, I found that the bubbles in the line would never end, even after 30 mins of bleeding there were still big continuous bubbles happening.

 

I am really stuck for ideas.

 

Could grit have backed up into the MC allowing air to be pumped in?

 

Any ideas what to do next would be very welcome!