avensis rear caliper sticky?

my 2001 toyota avensis has just gone through another set of pads on the osr.the piston would not push back in, so after removal i found that it screws onto a central thread,it seems to 'unscrew' as it is pumped out but there seems to be no way of it returning or going back in when released.the caliper also works the handbrake [no seperate shoes] but seems to work fine.any tips would be appriciated.thanks.