A core, be it a caliper or any other is expected to be bad!!! Thats why you are replacing it.... So, returning it should not be an issue.

As to rebuilding it, although I have not done one, I was told by an indy who had rebuilt a few that they do not last as long. I am not arguing this with anyone in the forum. Jsut sharing what i was told.

I've only replaced one and that was on my 85D recently with a good, used one. The old, bad one is sitting in my garage and may atempt to rebuild it this winter. At which time I will be picking the brainiacs in the forum.

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the sooner you start... the sooner you'll get done If it ain't broke, don't fix it.. Its always simpler to tell the truth...
2007 Honda Accord EX
2007 Honda Accord SE V6
96 C220
97 Explorer - Found Another Home
2000 Honda Accord V6 - Found Another Home
85 300D - Found Another Home
84 300D - Found Another Home
80 300TD - Found Another Home
Previous cars:
96 Caravan
87 Camry
84 Cressida
82 Vanagon
80 Fiesta
78 Nova
Ford Cortina
Opel Kadet
68 Kombi
Contessa

He might be right. The condition of the groove for the seal is never all that great on an existing unit. The rebuilds usually attend to this area via media blasting.

He also might not want to expend the time............if he spends one hour on each caliper...........he might as well have bought proper rebuilds.

The only time a core charge will be refused is if you hand the guy a caliper that is not repairable by visual inspection, meaning if the thing is cracked in half, or one of the ears with a bolt hole is broken off, etc. Other than that, the previous sentiments noted are correct - you are replacing it because it is broken.

The rebuild guys will turn down/grind/polish the pistons and rechrome and final size them, along with the cylinder bores. For a proper rebuild these tolerances are beyond what anyone is going to do without special tooling. They recover most of the returned cores as very few have real damage that won't be addressed by the rebuilding procedure.

I have used a "C" clamp to break a piston free in the past. Just be careful you don't punch through the caliper wall (that will make your core charge non-refundable). And, rebuilding works if there is no damage to the area that contacts the rubbing seal, the square cross section rubber ring that goes in the bore groove on the piston. If there is, well, the fix doesn't last long,

Good luck, Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

I also suspect that the improper use of petroleum-based solvents is a factor in the premature failure of field overhauls.

The question is always the amount of the "damage". None of those grooves are pristine after 20 years. There's always a bit of corrosion and surface contamination. Sometimes you can clean it up somewhat..........sometimes you can't. But, none of them are as good as a rebuild. The do manage to get that area quite clean. I'd be curious to know the required surface finish for long term durability of the seal.