First I would try to bleed all the brakes. Then if that doesn't work Take the calipers off and disassemble, clean and put back together. If that fails buy a new caliper. 'Nuff said. Mercedes has THE easiest brakes to work on.

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'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R