The look like solid cores. Hopefully you find it is something simple, like a crushed O-ring.

Perhaps they just sprayed some silver paint on them...

 

Same just happened to me. Same spot on both front calipers and the rears had different issues. One rear bleeder valve leaked around the threads (it just would not seat properly for whatever reason) and one of the pistons literally came apart on the other rear. It was a mess frankly.

The vendor was quick to exchange them, but it was a bit bothersome that a very reputable supplier had a whole set with issues. I've done three complete brake replacements and it certainly is not the norm. FWIW, all were Delco rebuilds. It's a PITA but now everything is all good!

 

!@#$%^&*()_ *******UPDATE******** _)(*&^%$#@!

The front driver side cleaned up with brake cleaner. Probably not leaking. but the passenger side still was leaking. I rebuilt the passenger side with the kit from Napa.

All the parts inside the brake caliper looked fine visually. No pictures, because they looked perfect. I replaced them anyway, being EXTRA carful when putting in the new seals. I lubed everything with brake fluid that went in the cylinders. However I did NOT lube the o ring with brake fluid. I made sure that was extra clean with absolutely nothing on it around it under it. My thinking was that if there was a path for brake fluid to pass, it might allow more to pass. If the o ring was dry when assembled, it may seal better.

I bled the brakes this morning and the car not only stops better, but the pedal has less travel than ever before (probably what it was always supposed to be).

I'll update this thread if anything goes sideways, but right now the thing is great.

On a side note: I started to hear grinding from the passenger rear. Only when turning left. Took the wheel off, and the cotter pin from the retaining pin for the brake pads had abandoned ship and the pin was half way out. This is the SECOND time this has happened to me. I thought that the previous owner had done it wrong the first time. And since then a professional mechanic had replaced all 4 calipers. So, does anyone use something other than the stock cotter pin to retain that brake pad pin?

 

Excellent..my bet is your pistons were leaking...
last time i heard scrapping frm rear was e brake parts letting go...suggest a look see there too..

 

**** like this is one reason I sold my C3 back in the day....never could get the brakes to stop leaking

good luck