The pulsation system relies on precisely timed air flow to open and close the liner around the teat in a consistent rhythm — that timing is controlled by a set of small valves that open and shut in sequence as the machine runs. Like any component that's moving constantly through every milking session, these valves wear with use, and as they wear, the precision of that timing starts to drift.
Because the valves in a set generally see the same amount of use and wear at a similar rate, replacing them together as a set — rather than waiting for one to fail and replacing it in isolation — is usually the more practical approach, and it's why we supply this as a set rather than individual valves. A drift in pulsation timing doesn't always announce itself obviously; it's often a gradual change in milking speed or rhythm consistency rather than a sudden failure, which is why periodic replacement as routine maintenance matters here.