First of all I'd fully strip any calipers your intending on fitting to your car as a matter of course anyway, just to give yourself piece of mind.
Also by fully stripping the calipers back to each of their key components, you can inspect each part of them in tern and check they are all ok.
The cast iron caliper body's are pretty robust items, so are generally alright, apart from being covered in dirt and the odd bit of surface rust, (which can easily be cleaned off), check for any cracks or damaged to the body of the caliper, especially if you got them from a scrap yard as the chances are the car they were taken from, was involved in a heavy accident which could have caused damage to them.
What you really want to do is check the condition of the actual piston within the caliper, don't worry about the inside of it, as these always go rusty, but carefully inspect the outside of the piston, as this is the part that the caliper's fluid seal bear's against. Any inperfection in this surface could cause fluid leaks and loss of breaking, which is NOT good.
Also check the condition of the rubber fluid seal and the rubber dust seal. Give them a gental stretch, (not too much as you dont want to damage them), but just enough so you can inspect them for cracks. The rubber should be soft and flexable, however if its not and there are tiny cracks in the surface of the seals this means the rubber is old and should be replaced.
I'd also check the condition of the rubber boots that go over the sliders, again these shouldn't have any cracks and should be soft and flexable. While your their check the slider rods and the holes they go into, they should be clean shiney metal and rust free. Also try and get all of the old grease out from the hole and replace it with new as this will help the caliper slide more easily.
Oh and I'd also replace the bleed nipples for new items, they're only coppers to buy anyway, so their's no real reason not to replace them. It also makes bleeding the brakes a hole lot easiler when you come to fit the calipers onto the car.
Sorry about the long post
, but I hope that little lot answers your question.
Brian
