Wippen rest cushion (or alternative rest rail) assists return-from-strike reset of hammer, key, and wippen by limiting downward travel of shank and bouncing it back into position. Maintaining about a shank's diameter of clearance between cushion and shank assures hammer weight is fully supported by jack and repetition lever with a little leeway for future settling of materials. When shank comes to rest on "rest" cushion, lost motion (or at least lost bearing) loosens wippen-to-knuckle linkage, reducing aftertouch and eroding key height, hammer rise, and backchecking. So keeping shanks off cushions is a starting point.
But cushions also function as stops, preventing shanks and tails from tapping or jamming and rear hammer shoulders from bumping on backchecks. Individual-cushion-to-shank gaps increase as hammerline gets turned up to accommodate hammer wear/filing or strings that are higher than spec (from plate crown or agraffe/V-bar wear/filing). Past a certain point, you can find backcheck buckskin on rear under-shoulders of affected hammers. Cutting pieces of action cloth and gluing them onto wippen cushions can mitigate (see photos below) and also improve function for not-tall-enough cushions in new wippens.
Another hazard of plate crown/raised hammerline (a version of which may have been there since the piano was new) is that tails of affected hammers will not be long enough and their backchecks not tall enough for good checking. Symptoms will show up mostly in the middle of the piano and high bass where strings are relatively the highest. Time-burnished tails will make matters worse, so a light tail-checkering might join the prep list. Just roughing, however, will not solve lack-of-reach geometry. If not replacing hammers, raise backchecks as needed. If replacing hammers, consider custom boring to eliminate string-height discrepancies. And make all tails the same length. If the original backchecks were shorter in bass and higher in treble to coordinate with original tails of tapered lengths (longest in bass, shortest in treble), lift and lower backchecks to all become the same correct height.
How? First of all, straighten backcheck wires. This may have already happened to some degree through vertical traveling and squaring. A WNG backcheck installation kit (or something similar) serves for pushing backchecks in. For pulling out, cobble together a slotted piece of hardwood on a simple base to extend over and just clear back ends of keys. Note (in photo above), this jig supports use of a pry bar to lever head and wire. The two jigs may need to be used in concert as it's hard to get the initial push or pull to not overshoot. A drop of CA glue to reinforce the new position may be appropriate. Then, re-square and re-space to finish.
Next, consider bolstering to restore old knuckle or wippen heel shape or to tighten up new part cloth or buckskin that came looser than desired. To reduce the possibility of noise, glue is kept away from potential contact with capstan or jack tip, leaving an unglued area for the bushing cloth bolster. Swiss Army Knife tweezers are perfect for this operation. Compressed, they push through, open up, and reach over the strip of cloth. Pinching cloth firmly between the tines, push-pull it back through to then pull cloth by hand until the previously cut end is flush with side of heel or knuckle. Finish with flush-cutting pliers on the other side and you're ready for the next.
Note in the lead photo a before and after comparison. Not only did the cloth become taut and more responsive, but it also became a shape that better-suited the action's slanted capstans.
Other potential extras might include filing knuckle buckskin for a more stable re-spacing. Cleaning and lubing wippen springs (and cleaning spring slots if off-rail). Sandpaper filing and wire brush combing backcheck coverings. Or filing and combing key end felt. Presenting such enhancements to the customer, particularly at initial inspection, may bolster pay for them. But some pay their own way by saving overall time and reducing annoyance.
Next week: Begin Regulating with a Plan