Rear caliper wont bleed

[midden]

I recently had to remove one piston and clean because it wouldn't push back in. A ring of crud had formed around the piston. 
I had trouble with bleeding the rear brake as well even though I was using one of those one way valve tubes from Halfords which I found crap. I ended up doing it the old way of closing the nipple before releasing the brake pedal after each pump and that worked a treat