Verdigris-coated end of the repetition spring binds in the repetition lever slot, making repetition erratic
With time, dust and wool fibers detach from the hammers and get between action parts and between the key frame and keybed. Aside from being allergens, dust and wool are abrasive, and should be vacuumed at least every few years to reduce noises and wear. Once the action cavity, keyboard, and action have been cleaned, contact points should be lightly lubricated. If action parts are sluggish, their center pins and bushings should be sized or lubricated.
In some Asian pianos, parts become sluggish because the center-pin plating cracks, flakes off, and becomes buried into the surrounding woolen bushings. Such parts can't be freed with lubrication, and are best replaced.
Even the highest quality action parts warp and twist slightly when exposed to humidity changes. This causes the parts to get out of alignment with each other, and wear unevenly. The tone can be affected if the shanks twist of warp, tilting the hammers and throwing them out of alignment with strings. Even when realigning the hammers is not an option (for example because the hammers are grooved and the owner doesn't want them reshaped), it is advisable to align the keys, wippens, and backchecks. This will ensure the most efficient transfer of force, and will reduce wear and prevent the parts from rubbing against each other.
A regulation of the action provides many benefits—excellent control in soft playing, effortless and well-controlled repetition, and uniform mechanical feedback. What it can't do is ensure an even resistance and inertial response from key to key. That's where touch optimization comes into play.
When hammers and action parts wear out and deteriorate, it is best to replace them. Preserving the original action parts is justified if the piano has historical value or if replacing them is not feasible for financial reasons, but keep in mind that old parts are brittle and even if rebushed and repinned they will never have the resilience of new parts.