Abutments are often heavily corroded.
Check for steps in the abutment that can snag the pads.
Carefully dress the abutments with a wire brush, emery cloth or file - be careful not to remove the core metal.
If the abutments are cleaned up without steps then the carrier is ok to use.
The Air Chamber face seal is important (1) - it is relatively easy to see if it is OK or not. Seal height should be at least 3mm.
The internal push rod seal (2) is very important for keeping water out. It is relatively difficult to check if it is ok or not!
Note that the front chamber of an air chamber is open to the atmosphere and internal parts can corrode (3).
Dust and debris (4) can indicate an air chamber sealing problem.
A tell-tale sign that the push rod seal/boot has split or is not sealing correctly is when water and rust staining is present on the pushrod (5)
ANY WATER TRACKS ON THE PUSHROD MEANS THAT THE AIR CHAMBER MUST BE REPLACED OTHERWISE THE CALIPER WILL FAIL
The pushrod and inner boot should always be completely clean and dry (6) - if not, the chamber must be replaced.