Air chambers have a pressure side and non-pressure side. The non-pressure side (closest to the caliper) has to be vented to atmosphere, and hence water can be present here also. On a drum brake this is not an issue as the pushrod is detached from the internals of the brake. However, for an Air Disc Brake, the pushrod has to be fully sealed against the non-pressure side of the air chamber so that water is kept out of the brake. If this seal is damaged, or ineffective, the brake will quickly become permanently damaged (through corrosion).
So, what happens if the air chamber seal is damaged or ineffective? The following photographs show the resulting corrosion after brakes have been fitted with an air chamber with a damaged seal:
More normal is for water to enter the brake and be undiscovered for weeks or months, until brake efficiency reduces, which is then picked up on rolling road inspection or test. This brake is completely beyond repair and had been fitted with an air chamber with a damaged seal.
Preventing Water Ingress
2. After checking the flange seal, carefully inspect the visible part of the pushrod. If water or rust staining is present on the pushrod, it is a clear sign that the pushrod seal/boot has split or is not seating correctly.
If water or rust staining is present on the pushrod (see image illustrating point 2), it is a clear sign that the pushrod seal/boot has split or is not seating correctly.
The inside of the boot and the pushrod should look like the image below.
Following the steps above will help preserve the life of calipers on trucks, trailers and buses. The caliper and service chamber interface is often overlooked, but it is always worth taking a few minutes to check the seal carefully to determine if a new air chamber is needed or not.