Ford Escort 94 escort diesel osf brake binding but caliper and piston are free to move!

94 Escort diesel OSF brake binding but caliper and piston are free to move!

Hi, I`ve got a 1994 diesel Escort that has the osf brake binding.
I changed the pads recently and thats when I noticed the binding.
The nsf pads were 1/2 inch thick and the osf were on the metal.
When replacing them the piston on the os would not go back and was "sluggish" to come out the ns would go back/out as normal.
"Siezed piston" I thought so popped it out but it was fine Huh

After boxing it all back up and bleeding the system the piston was the same as before BUT undo the bleedin nipple and it goes back ok..

[b]Hydraulic lock?[/b]

The binding doesnt feel too bad but after just a few miles the wheel is cooking...
Any ideas very welcome and appreciated..
Thanks Kev..