It could be that there is some corrosion in there making it harder for hte piston to come back.
A caliper rebuild kit is pretty cheap locally (circa $30 IIRC) and a pretty easy job if you are a bit mechanical.
At least first, give it a bleed and see if any junk comes out. If its very dirty fluid, I would be doing a caliper rebuild. If not, perhaps look more closely at the handbrake lever on the back of the caliper.
Check the condition of the pads on each side of the rear - of the other side is notably more worn, the piston may well be partly siezed in the bore.
'07 Touareg V6 TDI with air suspension
'98 Mk3 Cabriolet 2.0 8V
'99 A4 Quattro 1.8T
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