As previously discussed, the valve chests of many models of player pianos are a sort of “box” made up of two or more long boards. The secondary chest in my piano is typical in that it is comprised of a valve board and a pouch board. The two components are necessary as a valve cannot operate without its pouch.
The pouches are made of very thin leather (appropriately called pouch leather), which is cut into circles of a given diameter and glued in a “dished” position into their respective wells, which are simply precision holes drilled into the pouch board.
As with other components of the action, the decision when rebuilding is to replace or refurbish.
And as with other operations in player piano work, opinions vary.
Some say that replacing with new is the only way to guarantee long term reliable and uniform performance. Others say that new leather is of inferior quality compared to the material of yesteryear, and that if the old leather is still in usable condition, then it can be resealed for continued use.
If you decide to replace, pouch leather can still be sourced from an organ supply company.
Even new leather should be sealed, according to some advocates. More on that shortly.
To replace the pouches, clean off all traces of old leather and adhesive from the board, and check the board to ensure it is otherwise undamaged, and still true.
While the board is “de-pouched” is also a great opportunity to renew the sealant in the channels from the edge of the board to the pouch wells. Thick shellac or thinned PVCE glue can work well for this task. Bruce Newman recommends using cheap disposable pipe cleaners, of the appropriate diameter, for this task. It is important for each channel to be independently sealed, so as not to “bleed over” to its neighbour and risk ciphering notes.
The pouches can be glued down using a pouch setter, and hot hide glue is once again the glue of choice. Note that some pouches (like mine, as pictured above) have cardboard or fibre discs glued in the center. If these need to be replaced, use only the minimum amount of glue necessary to securely attach them, and glue the discs before the sealing takes place!
It is important to note that the two important characteristics of pouch leather are airtightness and suppleness. Ideally this membrane would be infinitely strong, airtight and flexible, but in reality there is a balance to be struck in the physical properties of the leather. In other words, in seeking to make the leather leak proof, we must not diminish the suppleness by introducing stiffness with the application of a sealant.
Traditional solutions for this are to use rubber cement diluted to 50% with thinner, or to use neatsfoot oil or a derivative product. More recently some folks like John Tuttle have been experimenting with silicone diluted in a solvent carrier, which you can read about here.
Once the pouches have been dealt with, you are likely looking at replacing the perimeter gaskets as well, or at least you might as well when you are at it. With your newly spiffed up pouch board, you are now ready to test the secondary chest!